After looking into the paperwork side of things, I think I'll need to mock it up into a stand-looking bike in order to be able to get a registration document. When I say 'standard-looking', it'll not have to look like the original XS, but like a 'normal' bike - ie rear suspension, mudguards etc. I can then get around to cutting it around properly after that.
The advantage of getting a registration that ties in with the age of the original frame and engine is that it'll not have to have fripperies such as indicators, and once it is 40 years old, there's no requirement for the yearly roadworthiness test (MoT) or road tax...