Yeah, I love my Garmin for training, as it stops me from cheating or taking short cuts, but I can't say it has ever helped me out in an event, whereas a decent waterproof and some gloves were the difference between finishing and a dnf in my last hundred.
The problem with flat ultras is that the walking doesn't take care of itself, as if you're not disciplined about it, you'll leave it until you have to walk, which is too late. Much like nutrition, the key to walking is little and often and from the start.
Martin, the Thames Trot follows the second half of TP100 (in reverse), although due to the time of year it is often rerouted. SDW50 is a great event, not a bad idea to do a 50 on more difficult terrain, as it makes the first half of the 100 seem easier.