Here in the US they called them Type S bags which could be gotten in both HEPA synthetic (which was what the supplied ones were) or regular paper bags.
They may be an uncommon one in the UK, as I'm not sure you got any other Hoovers that took those bags. Here it was mostly the full size canisters like the Anniversary Windtunnel canister, and going back further the Futura and Spectrum, etc. I don't know if any of those were ever sold there, I'm thinking not.
If you truly cannot find any Type S bags over there, what I might do is cut the collar off of a used bag, and glue it onto another bag that will fit the machine. Then you can use a more readily available bag instead.