Skippy is dead right. The return spring screw does occasionally stick fast to the return spring tube so get a but brutal and just unscrew it. It WILL unscrew, even if it unscrews the return spring tube from the trigger mech housing. When you replace it, make sure that the start of the thread is chamfered and replaced with a good dollop of graphite grease.
Nice to see that L1A1 Kevin the Kiwi. Does it still have the original long gas cylinder, one piece firing pin and sharp edged cocking handle slide that would put two sharp cuts down the left side of your hand when you let the cocking handle go