Can anyone point me in the direction of a supplier of a brass radiator cap flange that I can silver solder on to other piping as required.

Car Builer Solutions do an alloy version if that helps

just unsolder one from an old rad with blow torch

Also take one of the stub pipes from the rad using the same method.

Then solder the stub pipe to the base of the flange to give you a simple method of connecting it to a "Tee" fittling usinf bit of rad hose.