Having given the car another good run today I am now waiting for it to cool down to make sure this was in fact the root cause of the problem.
Watch this space -- as they say!

The fixing of the tank was actually the hardest bit, identifying suitable brackets to provide the supports, in the end I decided to use some 3mm steel brackets from B&Q. I have yet to fashion one for underneath the tank, but it is well supported by those fitted so far.
A major headache was reducing the size of the hose from the header tank from 25mm to 15mm, as the alternator had been relocated to the nearside this left little room for my initial plan to use a straight reducer, I solved this by using a 90degree elbow reducer immediately below the header tank, allowing me to use the 15mm hose behind the alternator to connect to the water pump.
After a good test yesterday running the engine up to temperature and giving the Stag a good hard run for half an hour, no leaks etc were found. All I need to do now ids fit the lower support bracket and maybe think about fitting a water level sensor. Round Britain Run preparation continues!