I'd highly recommend the Garmin 310xt Does pretty much everything you could need and will save you buying a lesser model now and upgrading in 9 months. It is Garmin's last-generation product which is why it's less than 1/3rd price of the newest, prettier watches. Absolutely packed full of features, very durable and easy to use and the ability to display 4 data fields at once is a winner for me.
You'll get a used one sub-£100 on ebay, a refurb about £110 and brand new for about £125.
DCRainmaker review of the 310xt will tell you all you ever wanted to know (and a lot more)
I am also contemplating this race and looking for a 4 hour time. I'd like to find some more reviews and hear opinions about the race so I will follow this thread with interest.
I have one previous marathon under my belt. October 2014 Leicester, 4:15. Elevation gain 164m. so pretty flat. Well organised but rather quiet and lonely with little in the way of supporters. It would seem Shakespeare is much the same???
I have seen on someone's mapmyrun trace that it's 84m of elevation gain, which seems flat, yet the few reviews I can find suggest some hills so perhaps that's not correct Does anyone have a garmin trace or similar that they would share?