Thanks @MagnusTS , that's a generous offer, but because of the thickness of the washers plus seals I need to convert the Cygnet's odd shaped hole to a normal one, I need 45-50mm length. Appreciate the offer, though.
OK no worries, looks like you've got quite a few suggestions of places to try for the 50mm one. My ball tap and spare nipple will be up on the for sale thread soon. When I get around to taking some photos.