I eventually went to my local hardware store, buying off t'internet is fine, but sometimes you just want to have whatever it is your after in your hands, without having to pay postage or going to collect.
The guy sold me a bottle of Kilrock K. Did the job today. The mineral deposits had been on the glass for a long time, coming from a permanently overflowing gutter that ran down th eside of the property and over one of the windows. So the deposits were tough. This stuff, Kilrock, did the job with ease, the whole pane was covered but it simply wiped off with a dab of the stuff on a cloth.
£2.50 for a 250ml containing formic acid.
Well impressed. Keep it in the van from now on and might stock up on a few bottles, it is cheap enough after all.
Matt