Philip.
There seems to be a common problem on the MGF whereby the engine will spin but not start. Several threads on this have been raised on the usual forums for our cars recently. The most recent is on the MGF register forum with the thread 'Non starting MGF'. The guy fixed the problem by replacing the 5AS, Mems ecu and key fob set. He is willing to donate to you his old set of bits if you think it would help you identify what is going on when this fault occurs. It would be great if you could narrow the issue down to it being a faulty 5AS or bad MEMs. When i did a search for this problem it seems several fixes have been made by just replacing the 5AS. Is there a code it sends the MEMS ecu that allows the engine to spin, but not fire, or just fire for a second or two?
If these parts are of value to you can you please contact him on the MGF register. If you are not a member then please let me know and i will contact hem directly.
Nigel

Thanks, I have sent the fellow a PM.
It seems more sensible to figure out what the problem really is rather than just keep throwing away perfectly good ECUs. Eventually they won't be so easy to get hold of.