I've made another couple of little mods that seem to make the skimmer run better. At John's suggestion, I've drilled two 2mm holes in the lid of the skimmer cup (through the centre moulding marks) so that the air-pressure doesn't stop the bubbles travelling up the flute, and I've also shifted my Phos-ex bag to the downstream side of the skimmer to try and keep the water level high in the skimmer chamber (it dropped when I removed the bio-filter bag), so we shall see how it goes now.
I can also report that the 'cooling' fan option is useless. It has no measurable effect - far more effective to just leave the front flat open.
Anyway, more news as it comes to hand...