Sorry to resurrect this thread, but Guy kindly messaged me to ask if I had fixed it. He kindly suggested a method to free the screw, ironically I'm an ex mechanic and if it had been a car the cooks blow lamp would have been used! Any how, it's fixed, there is some free play before the pistol fires, and I'm happier that the barrel is not going to slam shut while I'm loading!
Thanks again to ggggr! David