Again this is not the actual answer to your question but look at this page on the Rimmer Bros website
Sump Plug
Got to it by looking at Rover 800 Late Sump - 2000 Petrol, you will also see other cars it fits which might give you some where else to look for actual size.

Or there are these people who list one for £7
Magnetic sump plug

My Haynes manual doesn't give a size
Good luck

Last edited by Tim Crump; 02/01/21 06:20 PM.
Tim
Plus 4 2018