[Healeys] BN1 heater tap at water pump

[Noel and Helen standfast]

Hi Listers

I am looking for a heater control tap for the original cast iron water 
pump on my BN1 and I can't find a new one in the catalogues of the usual 
suppliers, does anyone know where I can buy one or alternately is any 
one willing to sell me a good second hand one?  Also I need to clean up 
the thread in the water pump after drilling out a stubborn plug, the 
thread appears to be an unusual 16 TPI thread, is it 5/8 x 16 Whitworth?

thanks


Noel S