Bongo Fury

When I stopped steam started coming from the radiator, the heater was still working, I thought I would put a tin of Radweld in and it would be OK. Next day the temp gauge went off the clock and the heater blew cold.Is the leak in the radiator and subsequent lack of pressure the cause, or a symptom of another problem.

I am not sure that radweld is up to the job, I would get the radiator checked and all hoses for leaks.

You don't say where on the radiator the steam is coming from - also, was the water in the header tank boiling?