I recently purchased one from a seller on Ebay and it was a good fit. I annealed it before fitting and it seems nice and tight after one re-torque.

http://www.ebay.com/itm/BSA-M20-FLATHEAD-SINGLE-500-COPPER-HEAD-GASKET-1942-/162058613896?vxp=mtr

email (option): cas.vanderwoude@gmail.com

I still have solid copper gaskets for sale, they are lazer cut from C106 soft copper sheet 0.9mm thick and so do not need annealing before fitting. £15 plus p+p. Top quality at a low price.

Regards TTJohn

email (option): Jomichael@aol.com

I've sent you a pm
Thanks

All sorted now, thanks to all who replied,
Alwyn