thanks everyone.
What we tried was this:
1st stage: lie in there until he fell asleep - we have a daybed in his room - very dangerous for me!
2nd stage: sheet on floor by cot. Bucket by cot. read, prayers, lights out, cuddle on bed in dark, put in cot. Ds says 'I'm going to be sick' I say 'would you like your bucket', very matter of fact, and give it to him. He makes ineffective spitty noises (you gotta laugh) then says 'that's better' I put bucket down, tell him i love him and will be just outside with the door open, then sit on stairs saying, in a very neutral voice 'mummies here' until he drops off.
3rd stage: gradually drop sheet and bucket routine, reply when he says 'I want to be sick' 'no you don't, you'll be fine, mummies just here'.. etc.
Now: read, prayers, cuddle, in cot, night night, outside on the step etc. etc.
So you see, we really have been very scientific about things
For me, it's just breaking the association with the sick thing, seeing as he can do it on demand. I don't mind clearing it up, but I do mind him choking (although I think you may be right about his age twiglet). The few times he has been sick when we didn't know it's been indescribably sad, checking him at around 10pm and seeing him with his head lying in a pool of crusty vomit. He wakes up saying, all pathetically 'mummy, I've been sick', he hasn't even cried when he's done it at the time, just fallen asleep in it. It makes me feel terrible.
ames - he doesn't do it at any other time - not lunch time sleep, only evening. I don't know how he does it, either, but I've seen it with my own eyes, he's a marvel!
imo hv's are useless - I haven't seen mine for over a year. And my doctor - i went for my 26 week check up, and he looked at me like I was mad when I described some stuff that had been going on, then said' but your little boy must be 2 by now - surely that can't be affecting you any more?' He didn't even know i was pregnant, although I've seen him 3/4 times this time and he had my notes in front of him! i thought I might get him to refer me to a child pschycologist, though, if nothing else works.
Twiglett - I think your method is good - controlled crying DESPITE the vomit, not avoiding it because i know he'll vomit. I'll start tonight!