New stereo arrived last night and was promptly fitted. Very easy process. The physical connection and installation took far longer than the configuring of the HU in question. For anyone who's not done it, apart from the single din HU, the only other items I required (apart from tools), was a single din cubby and a wiring adapter, both available from MX5parts at sensible prices.
I went for an Alpine 92 mechless unit, which has the usual bluetooth, USB, phone connectivity etc. Nothing genuinely special, but then it was pretty cheap. Chosen mainly because I've previously had really good experiences with Alpine gear. I mounted the cubby above the HU, which allowed me to put the microphone into the cubby itself, after drilling a hole in the top (not visible from the seats). So there's no wires showing.
Sounds pretty good (I've never heard any car unit get close to a genuine home stereo), has plenty of toys, is now the same colour as my instruments, and as a benefit, must weigh 1/4 of the old factory fit item, always a good thing in an MX5.