Well, now our temperatures have reached the dizzy heights of +1oC, my screenwash has finally re-entered the liquid state.
I had a couple of litres of Halford's screenwash concentrate, allegedly good to -25oC neat. So I chucked it all in (well, not quite, it didn't quite take the second litre), and drove to the shop and back in the hope that might mix it a bit. How much good it'll do, just under 2 litres of that plus just over 3 litres of whatever-the-hell Andrew put in there, but it's got to be a bit better.
Rolfe.