Winsol Crystal Clear 550 - if this stuff doesn't get rid of it, it ain't coming off!
Ive just finished a two day job using Winsol on this place

Viakal didnt touch, it nor did B + Q Commercial Limescale remover, nor did this stuff

which is usually pretty damn good.
I found Winsol to be the product of choice in the end, although even that stuff there were still a few windows that didnt come up as well as Id have liked. However saying that I dont know if the deposits were limescale or a mineral deposit of a different nature.