It has taken a long time to pluck up courage to do it, but as a first step I decided to very gently squeeze two bays of the aileron with my hand seemer as you see here. I protected the paint with some old soft hardboard, and squeezed it very gently so I could just see some change in the reflection when I took the pressure off.
I was quite sure that I had not done enough but decided to add another bay each time I landed until the problem was fixed. So, today I flew, and with this gentlest of adjustment it is perfect. So, if you have to do this, go gently!