Many thanks to everyone for their help.

Patrick. Your description of the problem is spot on. When the plane was built, it looks like a servo in each wing was put in the wrong way round.
The easiest solution would be to turn them. Unfortunately the servo hatches are glued in & I would spoil the fantastic airbrush work. Using 4 channels may not be an option, as my transmitter is one of the older, cheaper computer models. The easiest solution ( maybe not the cheapest ) is to use matchboxes.

Regards
Jim