SUCCESS!!!!
I tried Barry's idea with my trolley system this morning to clean my own windows (it had been in my estate boot over a week and was all frozen up - pole, hose and all.) Didn't take long to defrost my system. The water on the window frothed up at first but then settled, and the droplets on the window looked just like normal from outside (and inside).
My windows have mostly dried now and dried spotless!
The first container of water was hot and the second cold - but I stress that the last lot of water was stone cold coming out so it wasn't just the hot water stopping it freezing as the hose was lying in snow as I was messing about.
Might not stop it freezing at -22 but who'd be working in that anyway? -5 will be fine for me tomorrow.
I think I'll put hot water in my container for the first container of the day tomorrow, then refill with cold.
Got a few 2.5 litre concentrated screenwash bottles from Asda, Asda's own brand (as Tesco was sold out) for £1 each. Bargain!
Thanks again Barry!