I would try flushing the cooling system through,especially the rad and underfloor pipes.
They should vary slightly in temperature and should be reasonably firm but still squeezable if that makes sense
I'm afraid I cannot help, but just as a matter of interest so as not to confuse others, or is it me who is confused, the cooling circuit diagram posted by mgbluestreak on page two shows the hot water entering the rad via the bottom hose and the cooled water leaving at the top. This I am sure is contrary to what I have believed for years (55 ish). Am I really going senile or am I missing something ???
That's the way they are on the early cars for some reason.
I managed to get home for a couple of hours today, so had another play. flushed all the pipes & everything seems to flow properly.
Strangely though I noticed that when the heater is set to cold & the rad fan comes the return pipe from the rad cools down quite quickly, but when the heater is on hot it warms up as you'd expect. So it looks as if the rad fan is Cooling as it should but the water isn't flowing through the system.
Really confused now