Did about 20 miles with no more apparent loss and no odd symptons but thought I'd have a snoop anyway and found this:
Looked like G12 Coolant on top of the gearbox and it looked like it was coming from the coolant temp sensor housing on the side of the head
So I bought a replacement - part number 06A 121 132 J and also marked 06A 121 132 AP - this comes complete with the necessary seal and is £8.27 + VAT
You can now see where the leak was coming from
and here's a view from the passenger side (airbox & MAF removed for easier access. The water & coolant was dropped before I started and replaced with G12+ 50/50 mix with water.
When removing the old housing I removed the 3 water pipes that are on it as well as the coolant temp sensor and O-ring. Then, I took the pipe from the SA pump to combi valve out for a bit easier access to one of the bolts that holds it onto the head.
Also, after doing a bit of research on vwvortex 1.8t forums I found that a possible cause for this leak would be worn lower radiator mounts. When my car was cold I grabbed the base of the radiator near the bottom hose and was able to rock it a good way back. So I got 2 new mounts and did these at the same time.
Part number 1J0 806 157F at £4.61 ea + VAT
Bit of a mare to get in without removing lots (bumper has to come off plus the bolt that holds the old mount to the car) but my radiator's solid now and doesn't rock.
Big thank you [b] to Lee Ingham at VW Blackburn for getting me the parts double quick style [y]
It also appears that SDobbs and Patch are having the same problems with their cars - we all have X reg AUMs so it might be worth a look if you're slowing loosing coolant. Hope it helps someone