Thank you all for your thoughts. It was another victory for bloody colic - a twisted gut almost certainly. We tried large doses of painkillers/Buscopan which looked -for a few miutes - as if they MIGHT be helping - but his heart gave out (he was 22.) He is very sadly missed.
I do get so upset when some people refer to their horse having 'just a touch of colic' - every colic can very quickly turn to something far, far more serious, and should never be underestimated. Those of us who have sadly seen how awful it can be know it must always be treated as a possible veterinary emergency. Again, JG - so very sorry for your loss
(And please don't think I in any way am referring to this particular case - JG is far too much of an experienced horsewoman to even vaguely think it was minor xx)