Finally started putting Cyclemaster back together this weekend. It turns out the rings were correct but needed material removing to clear anti rotation pegs - not what it said on the accompanying leaflet!
Anyway; rings fitted, piston re-united with 'rod and gudgeon pin circlips in, I didn't quite like the way one was seating so removed it again to check that I had cleaned the groove out properly. Ping! Off it flew across the workshop, I heard it hit a tool box, bounce twice on the floor then silence - no aural clue as to its final resting place. I don't know what your workshop is like, but I held out little hope of seeing the circlip again. Indeed, after a visual sweep followed by a bit of fishing with a magnet produced nothing, I resigned myself to loose assembling the engine with a tied on label to remind myself of the omission.
Just as I was easing the cylinder over the rings and down past the gudgeon pin I knocked a socket off the bench and saw it roll under the flywheel of another engine. When I bent down to retrieve it, there also under the flywheel was the errant circlip! Eased the cylinder up again, re-fitted clip and Robert's your relative. Had it not been late on Sunday evening I would probably have gone out and bought a lottery ticket to see if this good luck extended into other fields ;-)