Thanks again everyone. I like to work on the bike myself but I'm really short on time this week. Even so, will work through things systematically after work to try and eliminate them as it will drive me mad if I can't find a cause and it doesn't happen again.
I just took it out for a ride to get it properly warm. It started fine, was pulling strong like normal. There have been no leaks in the garage since this morning when I started it.
I left the bike running for 5 minutes when I got back, fan didn't come on (as normal), but I didn't get a red light or anything. Radiator (and all corners) was hot. The pipes in the corners were not quite as hot as the fins on the side of the engine, but I couldn't keep my hands on them for long.
The water pump itself (and hoses going to it) were hot. When I took the cap off the radiator (turned engine off and left a few mins) I got a good hiss for the first part of opening. I started the engine with the cap off and could immediately see loads of bubbles in the radiator. Will do punk's check opening the drain screw with the engine on tomorrow before I drop the coolant tomorrow to see how much flow there is
Oil looks clean in the sight glass. Coolant is bright green (was premix when I last changed it and haven't topped up with water since). I did put a few mills more of coolant in the reservoir earlier (until it poured out the funnel), and when I've opened the rad cap just now, it did piss some out of the overflow pipe, which I imagine was just because it was overly full.
The only thing I did differently to normal on Saturday was the long periods of motorway driving (I normally only commute 10 miles each way, but did 70 miles in the morning, left the bike all day in the cold, then drove home). When I started the bike to come home on Saturday, I left it idling for 5 minutes on the side stand as I was gearing up (normally I just drive as soon as I start the engine). It was definitely around zero degrees, and I was driving at 70 in top gear (for at least 15 minutes) when the light came on initially, I would expect the engine to be cool in those conditions.
Will try coolant change tomorrow (with pump drain check), radiator check and flush. Whilst the coolant is drained I will pull the thermostat and the fan switch and try heating them up - I have a multimeter for the fan switch so will look at the specs in the manual. Will also try bridging the fan to make sure it's still working.
Cheers