Thanks for the replies guys, and yes it did take an eternity to make. What I found most "fiddly" of all was one of the poppet type valves. For some reason I was never able to get it to seat/seal completely. With the design of the engine there is no exhaust valve. Therefore when either piston is on its way up it is actually compressing. If you have a valve that is leaking slightly (as I do) there can be a moderate resistance on one of the pistons while it is moving back up. Regardless of this the engine still runs fine (there is a video of it in the video forum) but it does not have much power as a result.
Chris