thanks for the advice. i only started running in cotober last year, so am not quite sure whether or not i overpronate. i will buy some new shoes though, and will go to a specialist shop, they should be able to tell. when i got my first running shoes there, they thought i was a neutral runner, but maybe i am not, and that's causing the problem. it's definitely to do with the impact that running has; i can cycle for 2 1/2 hours and don't get any problems.
i went to a physio, but because the muscles only start hurting after a while, i think it was difficult to assess what was the problem. she said i should do some exercises to build up the muscles.
the pain affects the whole leg motion (i think). when it starts hurting, it's just a dull pain throughout the right hip.