In reply to richardhopton:
Sounds very similar to a pain I get.
Firstly, , a physio is prob your best bet, but this is what I do to deal with it.
I self massage using a tennis ball under the leg whislt sitting, getting the ball to work the whole hamstring and can really get it into the area behind the knee (is usually easy to find the painful spot, then work that area daily)
Started doing squats in the gym . Not using a machine, just a Miths press or a barbell over the shoulders. Nothing too heavy, 45kg and then do sets of 10 squats. Thius has really helped by strengthening the glutes and hamstrings.
Note, only do these once you are pain free.
This works for my hamstring issues behing the knees which came about when I was doing a lot of cross country running.
Yours could be completely different so take all that with a pinch of salt