Or one or more DU bushes are worn inside the wings.

Ask Streifender for advice, replacing them isn't too big a deal.
http://www.streifly.de/home-e.htm

GG

At 00:35 10 September 2016, JS wrote:
Thanks, JJ. So lubrication is the secret!
Something like LPS1 perhaps?
http://www.lpslabs.com/product-details/560
Unsure if it's available in Britain.
Jim