After going into many many many charity shops and finding nothing but a fake Montblanc, one finally came up trumps.
A local market trader who I'd given my number to gave me a bell yesterday to say he'd seen a Parker 75 in a local charity shop, and he'd ask them to put it aside for me if I wanted - which I did, very much so! This morning I went along to see what condition it was in, what pattern etc, and it turned out to be a pretty nice Cisele FP & BP set, complete with both boxes, cartridges, and two converters, though sadly not the nib adjuster. It's clearly well used - there are scuffs on the adjustment ring - but other than that it's in pretty good shape. Heavily tarnished, which buffed off with a jeweller's cloth, a good rinse, and it writes really nicely.
It cost me £35 (about $60), which I don't feel was expensive at all, and it's gone to a charity that supports blind people living in London.
From parker75.com's information, I think this is a 1970's model - but I can't guess any closer than that. Would anyone have any advice?