Ross,
I would agree with you and I plan on skimming through eBay and a few more forums to find the options out there if I can (military IT doesn;t allow eBay!). However, following up on a couple of the ideas posted here, I found this...
http://www.themx5restorer.co.uk/mx5-speedster/
That is a road-legal conversion, not subject to IVA and sold by a legitimate and well-respected MX5-oriented repair and conversion company. There are others out there too.
Cut windscreen and no wipers, one of which might, and the other definitely would, fail the S/IVA regs. I emailed the company and they affirmed that it does not require an IVA, though they had not been to Mazda for comment. If they are skating the edge of legality I'd be surprised - legitimate and experienced companies don't take significant risks like that, it could shut down the business.
I have queries in with Mazda, and will post any replies/comment here if they come back with anything useful. I'm also going to write to DVLA for a comment, see how far I get.
I'm also weighing up buying a Mk2 SE with MOT in moderate condition apart from the rusty seatbelt anchors and heavy scrapes down the RHS (which will all be stripped off anyway!).
Thank you again for all the views and comments, much appreciated!