As a reference the thread on the heater valve us 3/8" BSPT, British Standard Pipe Thread Tapered.
No luck in hardware stores, Ebay was the answer. I must admit I was shocked at how little staff in even the specialist hardware stores know, BSP...what's that from a local trade stockist. must have visited them the wrong time. I found the same with nuts and bolts, basically anything above 12mm is specialist and if you want UNC ..forget it! EBay has been my saviour.
Dave