Right folks, the experiment is over and the results are in!
First off, a rear Ray with a 245/45 tyre on really won't fit into a roadster boot:
However, if you remove the space saver then there's loads of room!
That done, I moved onto trying to get one into the front as well. It's a smaller space than I remembered, however...
Ta daa! The wheel is wedged between seat and rear bin, and there's no way in hell that it's going to move during normal road driving (I wouldn't take my chances on a fast country road drive without strapping it in properly though, but for a boring schlep up to Bedford it'll be fine). So there you go: It's very possible to get two rear wheels + tyres into a roadster, you just need to be creative.