Well what I thought would be aneasy operation, given all the kind advice, has turned out to be a complete disaster and I've removed the remains from the frame of one complete set of decals; so only one set left !
It is probably the case that the varnish was not tacky enough and / or the stiff paper backing was too tough to bend and keep bent whilst I removed it. I spent a lot of time shopping for a suitable varnish, not the polyurethane or water soluable types but the good old-fashioned yacht varnish.
I wondered afterwards if it was only necessary to varnish the actual letters and not the complete decal. I telelphoned a decal manufacturer and was told that this was correct.
I then went back to Chirico who sold me the decals and he told me that the whole decal should be coated with a thin layer of varnish, not just the letters etc. ! He reiterated what he'd told me initially :-
Cut the decals out according to the guiding lines etc., then lift a corner of the outer paper to release a small section of the decal tissue backing.
Stage two is to lightly coat the whole of the decal with a thin layer of varnish and allow it time to almost dry, during which time dampen the outer paper so that it is more pliant.
When almost dry then line the decal up on the frame and press home very firmly all over. Then immediately peel off the outer paper and once more press home to make sure the decal has stuck to the frame.
Leave the decal for some time until the varnish has fully dried, then sponge off the tissue backing, at the same time sponge the whole area sparingly to remove the residue adhesive and finally dry.
All this sounds simple enough to me, in my experience it isn't, at least not at a first attempt !
Finally, the owner of Chirico told me to either be very careful with the second set [which is the only other set in existence that he is aware of] - or get a professional signwriter to do the job.
Another dilemna !