Thanks to the MG mechanic, (yes, there is one here, he used to work in England, he's only 30km away, and he's got a beautiful RHD MGB GT), I was given a contact for HelpReg spol s.r.o. This firm issues technical documents for MG, Austin, Mini, Rover, Jaguar and Landrover (they are the main importer as well). They are recognised by the Dept of transport here, so anything they say should be accepted.
I had to send a copy of my Reg document and by return of post I received an official document listing the correct tyre sizes, along with other information. The cost was equivalent of 84 pounds.
Sadly, when I went to the registration office this morning, they had a "computer problem" and couldn't process any documents. Hmm, try again on Monday.