Thanks for the comments -
Yes the pressure had dropped after the first fill. At the start there was about 20g of stuff in there, and was replaced up to 1900g (I think). Second time it was down to 90g and was topped up to the correct level again. The second time was a 'quick fix' on a friday to see what happened, with the comment that I should bring it back when (or if) it lost the coolant again.
I haven't looked behind the dash yet - again I've seen a link to how to get it out somewhere on the forum but does anyone know where it is??
I don't know for sure it has lost pressure again, but it isn't cooling so it seems likely. With coolant in it certainly operated the clutch on the compressor OK, and now it doesn't pull in at all (but it will if you force it). I don't know about the bypass though - how does that work?
I have no reason to believe this guy doesn't know his stuff - he's not a Quick Fat Fattie, but quotes £350 to change the condensor if it has gone - that seems bit steep for a 'simple' change with parts at approx £100 new.