A Gas Bottle Problem

My van is fitted with 2 gas bottles and I have them linked with a simple Calor changeover kit with a useful "status" gauge and a tee-piece which takes the two pigtails.  That was OK with the 5kg cylinders, but I have now moved onto the Calor-lite cylinders - more gas, same weight.  I now don't need to carry around two cylinders for short trips, but I need to be able to cap off one side of the tee-piece (20mm thread). Does anyone know where I can get a 20mm stop cap ?  Calor don't make them, and I've had no luck searching the web.