Well, problem fixed!
I took the button apart and it was clean but in doing so I looked again at the travel and found that the button was not supporting the arrow fully and as a result was bottoming. Why it altered I know not, but alter it did (well the symptoms anyhow).
I gave the spring a precision stretch with a pair of pliers... sort of ... "that should do it", reassembled the button, retuned and all was well. Gotta love the DX for its simplicity!
I think part of the problem lies in the Beiter rest as we swapped over a few weeks ago from the ZT because we could not stop it drifting out. The ZT, with its slight incline holds the arrow into the riser. The Beiter, being at 90 degrees to the riser, doesn't exploit gravity in the same way.
Re: the paper test. It is important to understand the situation as we are not all blessed with
GNAS approved ranges at home. When setting-up in the garden I don't have a huge margin for error... safe but...
ConsequentlyI get nervous about shooting soething that is unknown, which by definition this is; so, I run a paper test at close range to make sure that nothing is too far out before extending the distance for a bare shaft tune. I admit that the paper test does not fix everything on a recurve but, IMHO, it is good for close-range work and excellent for seeing the effects of clearance work.
So, thanks to everyone that pondered on this problem for me and all the suggestions.
Good shooting!