Thanks Stirling.
I think the z axis is on a 2mm screw pitch, it was a pre-built z axis system, presumed it was "normal" ?
According to hypertherm guru Jim Colt, at the same power level, on the same material, the 30XP cutter will give the best results, so for the present at least I am sticking with that system.
I will have a further play around with the THC system.