Thanks everyone. I went out today but was unable to do any testing. We had a working bee that went on all day and by the time I could have a fly, I was just too tired. So next weekend (or perhaps before) I will give the changes recommended a go. I have not yet managed to find a suitable head spacer. The bore is 22mm. Any one know if any common OS head gasket would fit this? I can easily get OS parts but Leo - I would not know where to start.

After we finished painting, digging, cementing, cleaning and carpeting today (our pits are carpeted!!) a friend was flying an unusual aircraft. He is using it as a trainer for his 15 year old son. It is a fibre glass fuse from a 40 size Extra fitted with a non standard parallel sided wing. A sort of home grown Something Extra (but more of a Something Less). It is towed around by an elderly Enya SS40. Flies really well and the Enya sounded amazing puttering around at about half throttle. Hope I managed to get the Leo to sound so good.