Many thanks Trevor. In fact the people who did the body repairs on my car had a set of BSP taps and we established today that the thread is 1/8 BSP, a tap run through cleared the problem easily, he reckoned there was just a nick in the steel thread which caused the problem.

I believe the thread is a British standard Pipe thread, it is the same as the old galvanised iron pipe, at a guess it will be around 3/8" 0r 1/2"BSPT, taps and dies can be purchased from plumbers merchants
An old trick is to buy an iron plug (male thread) and put two saw cuts into the threaded section, at 90 degrees to the other and gently screw this into the brass drain, it will help to realign the thread if it is not too badly damaged
This sort of damage is caused by either cross threading or over tightening the plug.
You can if need be go to the next size up , by redrilling and tapping , but make sure you have the right size plug before redrilling, plumbers normally have these in brass, the threads are normally tapered
It may help to check the thread by using the following
1/4"BSP thread diameter 13.157 mm or 0.518"
3/8" BSP thread diameter 16.662mm or 0.662"
1/2" BSP thread diameter 20.955 mm or 0.825"

Hope this helps
Trevor

Does anyone know what thread the small brass fuel tank drain plug is please, I think the thread on the tank is damaged and want to buy a tap to try and clear it. When I removed the drain with the tank out is was very stiff and the thread on the plug is stripped, a replacement plug does not want to go in, all good fun!