Thanks for the advice guys - just thought it might be something simple which Ford may have seen before and sent out one of their Service Bulletins on. Shouldn't result in complete failure, or be very common, or else the fault wouldn't have happened?
Starts first time every time, and no noticeable 'extra' vapour or other stuff being expelled from exhaust - in fact its cleaner than the AUII XR8 just replace (which used to splatter little black dots of 'soot/oil' on rear of car).
I have looked all over the engine, hoses, entry of heater hoses into firewall, overflow etc but there is no sign of external leakage. Also as mentioned earlier, no oil contamination (at least not yet). Temperature doesn't go up. Water usage - well in 700km is the difference between the 'MIN' and 'MAX' mark lines on the top tank - maybe 500ml to 1 litre? Meant to measure it last time, but just plain forgot.
Aside from this lil problem she's a great beast to drive - who needs or could use much more power on the road? Really does go well!
AUIII XR8, Manual. Herrod Motorsport bits