What is this on the window?

This window was fine last time, this time it's all stained and it won't shift. The photo posted below is after cleaning it very thoroughly with no nonsense degreaser. I'm wondering if it's limescale somehow. 

Any idea what it is and how to remove it? Maybe buy some vinegar?

If it's limescale then Viakal, bought mine from Morrisons, will remove it. If they've had work done on the outside above the windows it could be an acid and not cleanable. 

That is some serious staining.  Looks like what you might find if there was a leaking overflow pipe above it or someone has been watering a hanging basket nearby with a garden hose.

I normally tell customers that I won’t tackle it until the source of the problem is fixed.  Then I normally use a descaler like Viakal.  Some have reported good results using white vinegar. 

I agree with the viakal. I have literally just used it on a seafront hotel that had this kind of staining on a couple of windows and it came up like new and didn't take too long in all honesty
Great stuff

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Looks like it’s mineral deposits from the lead , if you have used a chemical cleaner on it the lead can cause staining to the glass , the lead looks like it’s been bleached by something ???

If it’s lead try the pink stuff as that sometimes works. If it’s serious lead staining then bar keepers friend is the only only thing I’ve found will shift it.


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Thanks for the input everyone. Popped to the co-op and bought Viakal. Applied it twice and it's removed 95% of it.

No Idea what it could be from though. I've done this house for years and never seen anything like it. I'll have to ask the custy if I ever see him (GC customer)