[capn sparkle]

As heading - is it a bugger to get off - or easy with Viro-sol and the like??

[PoleKing]

Not really.
Bit of fairy liquid and a good scrub with hot and I've found it's fine.

[Tadgh O Shea]

Use a magic eraser pad damp with water finish with microfiber and its WD gone.

[George P]

for this you will need - new glass and a hammer, start by putting hammer through offending window, replace with new glass, good as new