Gosh a treasury of advice.
I have to discount a few things. The LE is very rounded, and unsurprisingly the idle stall is very benign. So, I don't think that it's wing shape.
It's simply flown with a slip-ball, so empirical beta estimation only. But, it's got quite a torqey engine which requires a constant right rudder input (i.e. the thing keeps trying to turn left). I allowed myself slightly too little right rudder, so that it tended to turn left - and it still dropped the right wing at the powered stall.
Thanks for the confirmation that Toblerone strips aren't likely to work - I certainly couldn't stand anything like a 3kn increase in Vso.
I suspect that the problem may well as IM suggested be inboard LE rather than the tip, particularly since it only seems very slightly alleviated by the selection of flaps. I had a chat to a colleague at a certain manufacturer of high winged light twins who told me that they had the same problem, but it was solved by fitting a wing fence twixt aileron and flap to contain any inboard section stall. I'm going to see if this is possible to try out.
JT is quite right about tufting, it's just a bit of a headache when against a short timescale on a high wing - because the problem is almost certainly going to be on the wing upper surface not the lower. So I've talked to the "man who makes things" and think I am going to (carefully !) try a day or two of empirical fix-fly-fix, and if that fails, do the job properly as JT suggests.
I shall report back, in the meantime, thanks a lot for the advice chaps.
G