I posted I'd not had any problems with the fluid I was using, and heated jets. I bought some more yesterday, and now I know why it's staying unfrozen.
The stuff I have been using is from Lidl. It's Ã‚Â£5 for 5 litres, though yesterday they were selling an alternative for Ã‚Â£1.69 for 5 litres.
The Ã‚Â£5 stuff will not freeze down to -60C. At 50/50 with water, it will work to -20C. The cheaper stuff didn't say.
So that'll be why I didn't have any problems as I've been using the stuff neat.