and finally, Luigi Chirico has told me that I should have "finished" the decal, [immediately after removing the tissue paper and dabbing dry] with a wipe all over with diesel fuel - not petrol / gasolene.
It has also been suggested that neat varnish straight from the tin should be thinned down with White Spirit, varnish 4 parts, White Spirit 1 part.
All this is academic because now I have placed an order for the decal lettering to be produced as a self-adhesive option, same colour and font style.