Sorry. Was slightly exhausted last night, but wanted to let people know straight away.
So....about 12:30 I hear an exclamation from downstairs (no other word for it) saying "Chi!" in a rather happy tone. I assume I've finally lost the plot and am now imagining OH saying Chi's name, but decide it's worth getting up to check. (I'd gone to bed about quarter of an hour previously.) Start to walk down the stairs to be met by a very distinctive "brrppping" sound as Chi runs up the stairs at me.
He's skinnier, collarless (already remedied) and on closer inspection smells vaguely of oil. No wounds or damage as far as I can see.
Downed some ham and chicken (him not me) and then got on with claiming his sleeping spots.
I want to thank all of you, not only for the vibes but for some of the more private support.