Well a bit of google cross referencing it seems the seal comes up as 18mm ID and someone lists a pattern part sender as being M18 x 1.5 with the VP number beside so I've plumped for the M18 and keeping my fingers crossed.
I was pleasantly surprised to find the size of the V-belts printed on then as well as a VP part number, so they are being replaced while I'm messing about in the engine bay.