From what you've said, it certainly looks like the builders have skanked the owner out of a few grand by not having a builders clean done. Can you get inside to have a closer look at the windows? Are there sticker marks etc still on the glass. If so, the only thing I can suggest is that you take the new owner round, show him and politely tell him what's what - there's been no builders clean and you didn't quote for one. If it can't be laddered then the only option for a scrape 'n scrub clean is a cherry picker.
What a shame it's left up to you, but unless it's a big company, I can't see them splashing out a few grand for a clean. And I don't like, "but they're giving nothing away at the moment" either!!!
PS: Slightly off topic - I think there should be a permanent thread here about the ins and outs of builders cleans. Until I'm in a position to employ an in-house lawyer, choke, I wouldn't touch one with a water fed barge pole.