Thanks to Susan and Carla (and others - follow the links) for giving me a great laugh with their 'Rules for Historical Fiction'.
This gave me a few ideas for my own 'Rules For Historical Fiction (And So-Called Fact) Featuring Edward II And Queen Isabella':
1) Early on in Edward and Isabella's marriage, Edward is usually portrayed as ignoring the beauteous Isabella in favour of the appalling Gascon upstart, Gaveston. Isabella, madly in love with her handsome yet neglectful husband, must be distraught about this. As an alternative, however, you might prefer to show Edward enthusiastically consummating his marriage, in which case the fact that Isabella is only twelve will be emphasised. The main point is to depict Edward II as a pervert, either because he had sex with a twelve-year-old girl, or because he didn't.
2) In 1312, Isabella must be portrayed as the forlorn and abandoned victim of her husband's cruel neglect, as he abandons her at Tynemouth in order to take his lover Gaveston to safety. She must also be shown to be in dire peril from the pursuing Earl of Lancaster. The fact that Edward II almost certainly wanted to spare his young, pregnant wife the rigours of a 5-day sea journey, and that she was in no danger at all from Lancaster, her own uncle, must under no circumstances be mentioned. As ever, Isabella must be depicted as a victim, at all costs.
3) It must be taken as historical fact that Edward wanted to divorce Isabella in 1324, although no documents have ever been found proving this. Hugh Despenser, the arch-villain of all time, must be the instigator of this and all other 'humilations' of Isabella, to give Isabella a good reason for her hideous execution of him in 1326. Isabella is a Helpless Victim of Despenser's nefarious schemes (see also number 2)
4) Isabella must have an affair with Roger Mortimer while he is imprisoned in the Tower, 1322-23, despite the logistical improbability (impossibility?) of such an affair. If you have to change the date of birth of Isabella's youngest child Joan by a year or so in order to put Isabella in the Tower at the right time, that is perfectly acceptable. Showing that Isabella even helped Mortimer to escape is optional, but definitely preferred.
5) Edward II is abnormal for not loving Isabella simply because she is beautiful. Roger Mortimer, on the other hand, must fall madly and completely in love with her the first time he sees her in France in 1325. Mortimer's inconvenient wife is conveniently never mentioned (see also number 15)
6) Edward and Hugh Despenser's peace treaty with Scotland is a cop-out, a betrayal and proves their total incompetence. Isabella and Mortimer's peace treaty with Scotland proves Isabella's astuteness, statecraft and foreknowledge of the later existence of the United Kingdom. Not, as a cynic such as myself might think, that Isabella and Mortimer's greed had depleted the Treasury to such an extent that they couldn't afford a military campaign and had no choice but to make peace with Scotland, and were desperate to get their grubby little mitts on Bruce's 20,000 pounds.
7) Edward's (admittedly tyrannical) executions of his enemies in 1322 must be presented as the worst thing that had ever happened in England, as though nobody had ever been executed or imprisoned before. By contrast, Isabella and Mortimer's totally illegal 1326 executions of the Earl of Arundel and his friends, and Hugh Despenser's friend Simon of Reading, are ignored, or alternatively shown as fully justified, because Mortimer hated these men. (Apparently, it's acceptable to have men beheaded, simply because you hate them.) Isabella's role in the judicial murder of her cousin and brother-in-law Kent in 1330 is downplayed or denied altogether, and Mortimer blamed.
8) Edward II's 1324 establishment of separate households for his three younger children, a perfectly normal procedure for royal children in the Middle Ages, is usually portrayed as 'stealing' Isabella's children from her, as though she never saw them again (and as though a medieval queen would have raised her own children anyway). As always, your aim must be to increase Isabella's victimhood.
9) Hugh Despenser the Younger must be depicted as one of the worst men in history, a man who could teach Hitler and Stalin a thing or two about evil. Throw any negative character trait you can at him - a 20th century mass murderer would make a good model for Hugh. It doesn't matter if the traits are contradictory, like making him both girlish and weak but also a brutal wife-beater, or if there's no historical evidence whatsoever, like having him torture people for entertainment. The aim is to make the fact that Isabella feasted and celebrated while watching Hugh being disembowelled and emasculated seem Morally Justified.
10) Hugh Despenser's greed and land-grabbing can be made to fill an entire book, but the fact that Isabella and Mortimer's relentless and ruthless self-aggrandisement was far, far worse can conveniently be ignored. You should mention as often as possible, and in terms of the greatest outrage, that Despenser's income was 7000 pounds a year and he held lands worth 6000 pounds, but ignore at all costs Isabella's annual income of 13,333 pounds, cash grants of 32,000 pounds in 6 weeks, theft of the 20,000 given to England by Robert Bruce, and appropriation of any lands she fancied. If you do choose to mention this, make sure you present her enormous greed as an acceptable and fully justified reaction to her long and terrible 'suffering'.
11) Edward II must be depicted as a misogynist, who only rarely comes to Isabella's bed because of his hatred and contempt for women. This means you will have to ignore his natural son Adam. If you can hint that Edward was not the father of Isabella's children, all the better. The same with Despenser and Gaveston - just ignore the former's 9 children, and the 2 daughters by 2 women of the latter, and make them as effeminate as possible.
12) In 1326, Isabella The Great Victim is suddenly and miraculously transformed into Isabella The Great Avenger, come to save the people of England from their terrible suffering. She is totally in control and capable of organising and leading a full-scale military invasion, despite the presence of Mortimer, the greatest solider in England at the time.
13) It's best to just ignore the fact that most of the men who supported Isabella and Mortimer in 1326/7 rebelled against them only 2 years later, including Isabella's uncle Henry of Lancaster, Mortimer's nephew Hugh Audley and his cousin Thomas Wake. If you really have to mention it, depict the men as a bunch of selfish, whiny malcontents with no legitimate grievances against Isabella's rule, which is of course perfect. (On the other hand, any rebellions against Edward and the Despensers are legitimate.)
14) Anything said or written by Isabella, especially in the years 1325-7, is presumed to be the gospel truth. If she said her life was in danger from the Despensers (1325) or from her husband (1327), then of course her life must have been in danger. If she said that Edward was a degenerate pervert, then he must have been a degenerate pervert. The fact that Isabella was the daughter of a master propagandist (Philip IV) is assumed to be irrelevant here.
15) The relationship of Isabella and Mortimer is the greatest love affair of the Middle Ages, fully justified because of Edward's neglect of her, and not at all a political marriage of convenience. Edward II's affairs with Gaveston and Despenser are, however, perverted and immoral. Even from a 21st century viewpoint, apparently. How nice that we've come so far. Mortimer must always be depicted as a heap of testosterone who makes Isabella quiver with desire, as opposed to Edward, who shuns her bed (see also number 1 and 11)
16) Any contemporary chronicle either hostile to Edward and the Despensers and/or approving of Isabella is assumed to be 100% true. Any chronicle which dares to criticise Isabella can safely be ignored, as it's clearly only propaganda to blacken her name, written by a misogynist who couldn't cope with powerful women.
17) You are allowed to make up any silly plot devices which 'prove' the survival of Edward II after 1327. Implausibility does not matter. The main aim is to get Isabella off the hook of having her husband murdered. If you do choose to have Edward murdered in 1327, make sure that Mortimer is solely responsible and that Isabella, implausibly, suspects nothing.
18) You are probably keen to re-write Isabella as a feminist icon, a kind of 14th century Warrior Princess who embodied the qualities of every powerful female leader who has ever lived. However, you don't know what to do when Isabella does something that you find morally repugnant. No problem! Mortimer only exists in the story to act as a useful scapegoat, and your strong-willed, powerful Warrior Princess was obviously temporarily possessed by a Fembot, who meekly did whatever her man told her to do. Hint: if Mortimer is unavailable, Henry of Lancaster makes a very acceptable alternative scapegoat. Even if you've portrayed both men elsewhere as possessing no power whatsoever, it doesn't matter. Who cares about contradictions?
Are all the above elements present in your book? Congratulations! You're ready for publication.