Sorry for my delay seeing these posts above, and for crashing the thread, I was really just thinking aloud...
For sure the clutch is engaging, you can see circulation in the "window" (which may be a bad thing if it's supposed to be full) but it shows the stuff racing by.
In the cab, it cools quickly after switch on, but after an hour's drive it's barely noticeable. The coolness fades off gradually, but the next day (or after a good rest) it's back again, cool.
It's been a year since a new drier was fitted and re-gassing and they tried to trace for leaks but came up with nothing.
I'll just take it to them again...
Thanks guys