After much head scatching and time wasted, finally found out that the calliper will retract so far on its screw thread but then needs to have a push at the same time to allow it to continue down all the way. Its as though the screw thread that it winds back on starts to turn with you after a certain point ,and you have to put presure on it to stop this action, then the piston will continue to screw down into the calliper. Might not happen to everyone but its worth a go if you find yourself in the same situation.