Just had a quick look through the workshop manual, both circuits (Air Con & Cig Lighter) are supplied by a seperate fuses, but earth at the same point. The earth point (G148b) lives, I think in the bowels of the dashboard, above the fuseboard. If it ain't the fuses then this is where I would start looking.
Finally got around to pulling fuses today, and low and behold the A/C fuse was blown! Bought a new one, slotted it in, A/C works
Now the weird part. The cigarette lighter provides power again. I unplug the fuse, the A/C stops, the lighter stops power, plug it back in, works again.
This seems very strange to me. The green ring around the cig lighter has been on the whole time, so this would suggest that part is supplied by the horns (and cig lighter as described in the workshop manual). To me, this suggests the supply of 12volt DC (is that right?) at the cig lighter is on the same supply as the A/C and fans.
This would also tie in with why the fuse blew in the first place - the wobbly iPhone cradle.
Also cheers chaps for your assistance, and especially you Stixy - top man!