Some good background information there, Keir.
As far as the mentoring process is concerned, I see it as bringing an extra set of eyes to review the training as it progresses and hopefully add a few pertinent suggestions which may help the process along.
In one sense, the difference between 2:54 and 2:49 isnt much at all but then again we all know that an extra 12 secs per mile can become a huge mental burden if too much focus is placed on that rather than concentrating on getting the basic training components right. As long as focus is on the inputs rather than the outputs and that leads to standing on the start line at Greenwich confident that there is a good marathon campaign under the belt, then the performance on the day will be whatever it will be.
As for Hilly's relevant question of what can a mentor with a similar target bring to the party I think the most valuable resource I have is that I have managed to cock up marathons in just about every way imaginable (about 22-24 in total), so know quite a bit about what not to do. 
As a bit of background, I am old enough for my sub 2:50 clockings (2:48 PB) to now be adults in their own rights, having been run in the 1990-93 period.
I then had a little 15 year break and started again in 2008, as many do, at 5 stone overweight. I race quite a lot so have completed about 320 races of various sorts on the comeback trail with 7 marathons progressing as follows:
3:24, DNF, 3:08, 3:04, 3:18, 2:57, 2:55 with the last two being VLM this year and Chester 10 weeks ago.
At 46 I'm quite pleased with how this is going so far and want to see if I can get to sub 2:45 in the next couple of years and then again as a V50. If not, then at least I will have got a lot of enjoyment out of trying. 
So that we can concentrate on Keir's training on here I will be blogging my activities here, as I did for the two sub 3 attempts this year.
So best of luck, Keir. I'm sure the next 4-5 months are going to be a very enjoyable process.......
A couple of comments so far on what you have outlined.
1. A few years ago I would have winced at the thought of a 33 mile ultra in the build up. However, I would not have any such concerns any more. As an example, in the build up to Chester a certain challenge involved running 70 miles over 2 days 6 weeks out from the marathon.
Far from being detrimental, recovery was swift and led to comeback PBs at 5K (17:25) 6 days later and over a mile (5:00.2) the following week along with a boost in strength for the remainder of the campaign.
However, the key thing here was keeping the pace of the 70 miles of running under control. Too much stress in terms of pace would undoubtedly have led to a less positive marathon outcome so to use the ultra as a positive boost I would advise keeping a lid on the pace.
2. I note you ran a marathon at MP +60 6 weeks before Abingdon. If you manage to maintain mileage closer to 80mpw during this campaign I would suggest, maybe counter intuitively, that you try to move this session a little closer to marathon day, eg 3-4 weeks out.
Looking forward to seeing the rough campaign outline.....
Edited: 23/12/2012 at 18:23