Hi Nick
I had this problem a few weeks ago, same sort of trusty old shoes, bought a new pair from a good running shop with the usual checking for correct fit, type etc.
Problem vitually disappeared overnight. Most of the reading I did around this subject suggested that most shoes are only good for about 600 miles of running, in my case about 5/6 months. I'd look into this if I was you.
Goo luck Cha