Great News! After Clive very generously donated a flywheel from a Spit 1500, along with the clutch cover and bolts, I was able to source a 190mm 1", 23-spline driven plate, for £55 delivered, from this guy, Neil McCarty, who I found on the Facebook group "Type 9 Gearbox World" (who knew there was a whole Facebook group dedicated to just this gearbox?!)
After an epic job of replacing the gearbox, which after about 6 hours I realised I ultimately had to remove the inlet/outlet manifold as it was stopping the input shaft aligning with the cam shaft spigot bearing (in fact it wouldn't even go through the hole in the clutch cover finger plate) I now have a mobile car again. A couple of minor issues to fix, such as the speedo not working, so I must have made a mistake connecting the cable in the gearbox drive but altogether very happy.
Thanks again Clive and thanks Neil McCarty, if you're out there