Really sorry to hear that things are not good with your hamstring - but some answers at long last it seems!
I have now had my ultrasound and it shows a mass of musle which shouldn't be there in this particular place, but no damage to it and no apparent reason why it is still going into spasm.
Now waiting for consultants appointment.
In the meantime - physios have tried and now ruled out an inbalance in my back and this morning suggested it could be scar tissue that is causing it all when I saw a different physio.
So still playing a waiting game.