There has been so much to take in and so much great information and links offered that this thread is worth a bump.
I have continued my interest, although there is much else to absorb me, but I am "getting my eye in" and sold a couple of inexpensive items on eBay. There is tremendous competition in the jewellery market, much of it new, of course.
One thing I found was that the US term rhinestone (as in "Rhinestone Cowboy", of course) is probably more common and recognisable than diamante or paste. I have not yet pinned down accurate definitions, or indeed understood the differences.
I seriously believe I can identify a niche market for vintage jewellery specifically containing glass, as opposed to precious metal or gemstones. Following specific designers may be much more difficult.
As a measure of my commitment I have invested Â£75 in an ultrasonic cleaner and am practising my close-up photography
Thanks for your continuing interest and information.