Thanks for your input, everyone who contributed here. I thought I'd give an update because I've ended up doing something I never expected. I've bought a second hand virtual button accordion (a Roland FR7x) , despite being (initially) adamant I want to play a 'real' rather than a digital instrument.
But the logic is pretty strong - I just don't yet have the experience to know what I really want in any detail, and this thing can do it all - it's 5 rows, 120 bass, with all the different configurations available at the touch of a (number of) buttons. It plays beautifully. I've found a teacher to help me with free bass and am using it as stradella for my weekly ensemble sessions. It's currently set to B system but if I want to try C system with another teacher, it's just a setting away. I figure I'll use it for 6 or 9 months or something and then sell it on - or maybe I'll get used to it and like it, who knows?
A crucial bit for me was that the guy in the shop (Allodi) was really extremely helpful, and promised to buy it back for a very good sum if I take it back within a year, so I'm really not risking much at all. Whereas if I splashed out on a real instrument, I think the risk would be far higher.
I'm delighted and having loads of fun with it so far.