Steve Sir, I need corroboration on something. I recall you did a coolant system overhaul that included changing the radiator, around 2-3 years back. You posted up a very detailed diy on it with loads of pictures, as I recall.
You obviously bled the radiator after fixing everything up. Did you monitor coolant levels daily after that? How did it behave? Did it fall, and did you top up, and if so, at which point (i.e days or weeks) did it stop falling? Could you estimate the number of expansion tanks-worth of topups you had to do ?
As mentioned, when I changed my rad and bled the radiator both hot and cold, I still had to top up for nearly 3 weeks thereafter before coolant levels stopped moving from full, and I'm estimating that it involved 1.5 exptanks worth of extra coolant.