Strange that you say it disappears if keep going. I wore my old trainers yesterday, started to get the same pins and needles sensations but after around 10 minutes of trying to focus more on my running posture the numbness disappeared again. I think I am going to try purchasing new trainers after a review of my running posture at the store before going down the (potentially more expensive!) medical route.
Had ruled out the laces, bought a pair of the elastic lace thingies and it was still the same. Think I am going to have to go to the specialist running store next time I am in the city and get analysed on the treadmill