Hopefully this might help somebody. I am so glad there are people out there with the same problem as me. I never met anyone with this problem before and was beginning to think I must be abnormal...
Hav been going to a sports specialist this past few months and don't know if this, decreasing my miles or doing more running on grass is helping prevent the numbness. However, I am now upping my mileage again for London Marathon and am dreading the numbness coming back. I change my shoes every couple of hundred miles (though Mizuna's seemed to be the worst for me) and it does not help. I also have tried all the advice re loosening laces, wearing a bigger sized shoe, stretching out, etc.
My sports physio cannot pin-point the problem, but seems to think it may be something to do with the sheath/band at the bottom of my right ankle that covers the point where the tendon and muscle meet (I think I have got this right!). Apparently, this sheath protects the tendon and can sometimes become damaged. She says when you run (especially longer mileage) and increased blood pumps through your muscles, they expand causing the sheath to expand also. If this is damaged it may not expand and thus can compress the muscle and nerves, thus causing the numbness. By holding some part of my ankle/foot during one session, she was able to reproduce the numbness in my foot. She also said tight calves may be a factor as well.
When she works on the area around the tendon, it is extremeley painful but seems to be doing the trick (however, I also decreased my mileage at the same time which might be a factor). I was told after every run to stretch and ice this area of my ankle. She also said, to turn the water to cold when showering and run on my legs for as long as can be beared, which will help reduce any inflamation.