After looking at it further, I put a 13 mm socket on the nut of the impellar while holding the shaft the other end with a flat end screwdriver as I unscrewded, the pump assy.came off.
Cause of motor jamming was the impellar travelling too far up the shaft because pump assy. broken.
Fitted new assy. & been going OK for over a month.
Kim
June 2009