Hazy double glazing (not condensation)

[kryten22uk]

One of the double glazing units in my kitchen patio doors has a strange haziness to the surface, which very much looks like it would if there were some cellophane stuck to the pane. I've cleaned with a variety of products and tried a darn good bit of elbow-grease in one area but there is no change to the effect. Furthermore the hazy effect is not uniform, its very much variable.

Tried to show it in the photos below but not sure its overly clear to see.
Any ideas what it might be and whether solveable, or whether I'll just have to get the unit replaced?