Before anyone spends any time answering the questions about flushage procedures ... I found
this thread which seems to have all the info I'm after! :)
The needle was heading towards the Orange Zone on Saturday, not all that quickly to be honest. I was stuck in crap traffic idling for quite some time, which is when I noticed it was getting hotter than I thought it usually did, but once the traffic freed-up & I could dribble along at 80, there was no difference. Putting the HVAC fan on & opening the heater-valve helped pretty quickly.
Stopping at a servo (no apparent leaks or losses, radiator felt hot, top-hose hot & didn't think to touch bottom hose) & trying to squeeze the firm-feeling top hose had little response on the first squeeze or two, but I did it again & got a burp inside the overflow tank.
The rest of the drive I did see the temp needle starting to move up, but it'd invariably drop down again on it's own; I don't drive it regularly enough to know if it's normal for the temp to move between just under half & around 2/3 on the gauge (it's only just now occurred to me I should pull the logs & see what the actual temps were).
I've not replaced the coolant since I last pulled the turbo, which was probably over 3 years ago (but also probably only 500km ago at most). So my plan is to drain, follow flush procedures from thread above, replace thermostat & maybe coolant-reservoir cap just in case (depends on what they cost), and cross fingers a lot.
Removing the radiator is a bit of a PITA though, I'd rather avoid that if it's unlikely to tell me much! :)