Here is a thread with links to treating paper with gelatin, Canon gloss optimizer or Image Specialists gloss optimizer.Thanks to The Hat for that very clear advice. Thanks too to Ink stained fingers for sharing your experience and expertise.
Interestingly, a while ago now I did some extensive testing overprinting prints using the Epson gloss optimiser in a Canon (dye based) printer. The optimiser made very little improvement to the prints and the printhead was unaffected and is still in use today. Maybe that is because the optimiser is 95% glycerine and water - so perhaps nothing there to do any harm!
My search continues for something that I can use as a pre-coat or after-coat which will give a more glossy appearance and a greater depth of colour to prints which are printed on uncoated card.