And the disk no longer boots. Billy doesn't seem to appreciate the full magnitude of the situation, managing a 'sorry' in a 'whatever' tone of voice. Perhaps he's right? For £20 we could get a new one.
Anyway I sent it off to http://cd-repair.co.uk/ and for £3.50 they give the disk a serious polish and set it back within the week. Nice service. You can actually see the disk is thinner! But it works fine now. I don't think there's enough plastic above the data-layer for a second chance though.