One idea, this is the mk1 Fanmaster, it has a wall mounted control switch with five lights and a temperature dial
First though the Fanmaster is designed to work like this; the van comes up to temperature as set on the dial, it switches off the heat and the fan slows down, (on 'slow' fan the fan speed does not change much)
It should then cycle on and off to maintain the temperature automatically without particularly feeling a draught from the vents.
If that's not so then I think it's quite possible the internals of the Fanmaster and particularly the fan blades are coated in fluff, this leads to not enough air getting through the elements, in turn this will allow the elements to overheat and the thermal overheat trip is operating to protect it.
At the least the Fanmaster needs removing for a thorougher cleaning, you will only make things worse trying to do it in situ.
Another possible problem is a worn fan motor is not spinning as fast as it should with the same result as above. If it's noisy at all then this is a sure sign of wear that apart from tripping out, will seriously damage the control board sooner rather than later.
