Cheers all!
YD - Well done getting the miles in when you could. It all adds up. Regarding warm-up races, I'm still working that one out myself. I suppose I'm veering towards being happy to race quite frequently, but making sure I can prioritise A/B races and not let tapering get in the way too much of the bigger picture.
Bearing in mind it's your debut, I personally like the idea of a 20 mile race, if you approach it properly. It will give you an opportunity to run a very long race (IMO it's significantly different enough from a HM to feel like genuine marathon prep, but still within a reasonable range regarding recovery) as a proper dress rehearsal for the marathon - use your gel/fuel strategy that you're planning, and obviously race shoes, etc. I can remember discovering silly little things like, how easy is it to actually take the gel out of the loop of the gel belt when you're running at 6:xx/m and feeling a bit tired?...
(Without meaning to be too prescriptive...) running a 20 mile race 4/5 weeks out from marathon day, with a progressive easy/steady start and then 10 miles @ target MP, would be good preparation; 20 miles raced flat-out three weeks before the marathon obviously wouldn't!
Mr V - Good leg strengthening as you say, plus you feel quicker again when you get on the road! I think my XC form is pretty much on a par with other running; on the one hand, I'm aerobically fit and my legs are strong (on the stupid-steep stuff I can power up pretty well), on the other hand my running style is a little heavy, so I don't glide over the muddy stuff like some of the weedier (meant in the nicest possible way of course!) runners seem to. 
Simon - Good question. I don't know how much, if any, Paul had in reserve but it's possible I could've either opened a gap and clung on, or maybe had a bit more for a sprint if my rythm hadn't been so interrupted. I'll just have to give it another crack next year.