You guys were dead right. I adjusted the carb to a richer setting and it works much better. However I noticed that if it was rich enough to enable perfect running on a verticle ascent, then it's too rich when in a dive under tickover throttle, as the engine struggles and dies. I had to reach a compromise setting which allows near vertical ascent under all throttle settings plus a vertical dive angle under partial throttle to max throttle. As long as I remember to have the engine above idle on a vert dive it will now be fine.

However all the above testing was done with the fuel tank out of the plane, to eliminate the pickup issue's at extreme angles. The engine is now perfect, but I still need a better designed tank. I have in the post a sullivan 1oz round bottle and slant bottle to get me going for now, plus a 2oz Hayes from USA (which will take a while), plus hopefully Andrews MFA 1oz tank shortly.

Looks like I might even get airbourne at this rate! Thanks all