Where can I get this gasket?

Small block cars, where the hard water pipe joins to the back of the water pump. 

Or shall I just make one?

You have the metal pipe attaching to it, if I remember rightly?

I'm pretty sure I ended up making one. Hasn't leaked yet……… touch wood.

Alternatively

https://www.vwheritage.com/052121091a-water-pump-gasket-vw-spare

https://www.ebay.co.uk/itm/Genuine-VW-Seal-052121091A-/332470379813

Ian

That's the one, cheers buddy!