Duly researched it (thanks to everyone on here), sprayed the air duct sensor with some GT85 and off I went. Seemed to fix the problem for a day but then it started doing it again.
This time, riding through awful traffic on the M25, the EFI message came up but intermittently flashed back to the steadily rising temperature. Traffic cleared, problem went away.
Next day, after stopping for petrol near work (had ridden 60+ miles), the fans came on when I turned the bike back on! Bit weird thought I.
Anyway, coming home, the EFI message and rising temperature started to happen big time and hit 118c when I was filtering

So, checked the bike today and coolant was dripping out of the hose leading to the coolant tank. It looks like the clip (one of those horrible 'should use some special tool to get them off' things) was no longer gripping tightly enough. After much faffing around, breaking the old clip, trying a jubilee clip (too big), I've used a cable tie and am now waiting till tomorrow to see if it leaks...
Such fun!