This is a very obvious comment, especially in hind sight and thought I'd mention it here to hopefully allow anyone else following this route to a scalish plane to avoid disaster.
Both Kelvin and myself have chosen to use non standard exhausts. My engine was brand new and I fitted the exhaust and mounted it all into my workmate to setup and do the initial running in. By the time it went into the plane it ran beautifully. However after the first few flights with a rich ish mixture, the engine bedded in further and chose to stop several times in mid air at low throttle settings. Fortunately I managed to dead stick it in without any damage apart from a few more grey hairs!
I need to lean out the bottom end mixture and or fit a hotter plug or even on-board glow. So as I've said, its very obvious now but could have wrecked the plane. I expected the mixture settings to be different from standard because of the different gas flow of the exhaust, but didn't expect the top and bottom end settings to separate from each other to such an extent as the engine ran in.