Anyway, I thought the value comparison might please BX owners - compared to the available BXs, the Xantia I got for £550 (which is £50 less than I was offered cash for the MX5!).
The route to get there was longer than required, involving a Golf GTi and buying the Xantia outright first, but I got there - the GTi sold and with the cash from that and the MX5 deal, the Xantia's paid for as is 6 months tax.
Here's the Xantia:

1.8i LX with AC, 99,400 or so miles with MOT until August. Usual bit of rust in the rear door shut (thanks, Heuliez) but incredibly clean inside, everything works bar a couple of bulbs. Lack of use means it probably needs a good suspension service, but history includes £900 for work in July at a main dealer; the main dealer then reported that it's had a clutch, clutch cable, cambelt all done around 80,000 miles, and it even had brand new sunvisors fitted at a cost of £100 (vs trying to find a set for a tenner in a scrappy).
So that would appear to be what that money gets you in Xantia form these days; it certainly doesn't seem to get much of a BX. Looking on eBay there are a few 1.8i estates around the same price, though they probably don't have the history they also may not have the rust, or be nicer colours than white.