Thank you all for your replies, and apologies for not getting back before. I've had to change my e mail address as a hacker has been plaguing me for the past two or three weeks, and you wouldn't believe the hassle! Well, I expect you would!! particularly doing it on my Kindle!!!
Merlin is fine with being stabled, at least for the time being. He is such a lovely good tempered, laid back boy, and I just can't believe that this has happened as his paddock is only about 1/4 acre and very sparse. I can only suppose that as someone said the grass is particularly sugary this year owing to the warm wettish spring, and being out 24/7 was just too much for him. Luckily we have got a yard of sorts outside his stable, which at the moment is grassy(!!) and full of garden grot. We shall clear this, fence it, and he can go there, and any grazing will have to be done with a muzzle on. He seems a lot better today, standing more comfortably, eating well, pooing properly, and the crest on his neck is softer and less obvious. We are hopeful that he will recover, but of course he will have to be monitored closely. Again many thanks for all your comments, all very helpful.