There's quite a few stored Mk3 sleeper coaches at Long Marston that I saw on Sunday at their fantastic open day. Most are still carrying Intercity livery and the details regarding maintenance on the coach ends suggested that they last had any kind of major work in the early 90's.
At the open day last year, a mate and I did manage to have a quick look inside one and found a couple of Intercity Highland Sleeper leaflets. The interiors, especially the coach ends were in a pretty sorry state, although the cabins/berths themselves weren't too bad. Don't get me wrong, you couldn't sleep in it straight away (all the mattresses are removed and they're shabby and dirty) but nothing that a few quid couldn't sort.
But saying that, it did seem pretty clear that the "underneath" had been robbed pretty heavily for parts, and unfortunately, that goes for most locos and coaches there.
I did have a few photos on my fotopic site of the inside of one, but I've not got round to putting it on Flickr yet. If anyone is interested, I'll upload them when I get in from work later.