never assume because a part is new that it's not the cause of a fault, I've found that out too. Pleased it's sorted and thanks for sharing the conclusion too as most don't so when you search you don't find out what the fix is.
One of the worst things to find out is that a faulty part has been fitted. Glad is sorted.
I owned an old TR6 which had overheating problems. Changed just about everything related and in the end it turned out to be a new thermostat causing the problems.