After two phone calls and several 'please holds' totalling more than 30 minutes (!!!), I was told by a senior adviser that the retail install disc available from the online and actual stores would install on my Mac Pro — despite never telling me what version it has — so I could just go out an buy it (which I'm ok to do).
Or, he said, he could send me a 'retail' disc with 10.6.4 on it for free.
I took the second option 🙂
I expect he means the grey discs that originally shipped with the 5,1 Mac Pros, but I'll wait and see.