I should've looked this up on the forum ages ago, I've been worrying about my numb feet and 'stump syndrome' for ages and its good to know that loads of other people have this problem.
I've been trying to work out why, when I did and trained for the London Marathon 5 years ago, that my feet were fine, but since then I've had numbness during a lot of the road races I've done - all distances, from 5k to half marathon. They don't go numb when I train off road. The only thing I was doing differently when training for the LM was that I was doing a yoga class regularly, so perhaps the extra stretching I was doing then must have helped.