Martin, The guard you have bought is not from a Plunger A10, it is narrower, and of the style of the earlier A7 and Star Twin. Do a search for A7 Longstroke, see how the images compare with your mudguard.
Here is a modern one from a UK supplier for comparison.
https://burtonbikebits.net/images/CIMG1220.jpg This is the basic pressing, needs the support bridge mounting holes, but shows a similar shape to yours. You can see a strengthening plate tack welded to the bottom rear for the rear stay. Being a modern reproduction it may not be 100% accurate, and of course yours may not actually be a genuine original BSA part.
Swarfy.