Nightmare scenario: carefully ballasted round a point, only to find it wasn't careful enough and the thing would not lock off. Fortunately it's a Peco point with the spring block held in place by metal tabs. Opened the tabs, lifted off the bias spring and bock, and found a piece of grit in the middle. Cleaned, lightly oiled, reassembled, and it now works a treat. Three cheers for repairable!
Guy