I could have cried. Well, I did a bit. Bought him down for more meds and he ate his breakfast - gums a better colour etc. although the pee he had on the way wasn't much and was the colour of weak black tea. 
Quick update: he had nothing to drink for 24 hours going into last night, had a toxic line showing on his gums again and was starting to dehydrate. Vet came out and gave him another half shot of Finadyne (anti-inflamatory/painkiller) and some Finadyne paste for today and tomorrow and I went to bed thinking if he hadn't drunk by this morning, it was probably time to call it a day. Couldn't tempt him with any of his favourite treats.
Went up at 6am and he'd drunk just over a bucket of water
When I took him back to put him in his bit of the paddock (so I could monitor his fluid intake, as the rest of the field has a self-filling trough) he kept digging his heels in. I've had him 9 years and he tells me everything he's feeling as he's feeling it, so I thought: 'Fair enough. You want to go out with your mates, so because I know you've had some fluids, you can. But please, for the love of all the Horse Gods, drink something'. Bless his grubby little grey cottons, he went straight over to the trough and had a drink and he's been happily grazing the lovely wet grass all day. Never thought I'd be so grateful for the rain! Also scoffed some treats after his oral antibiotics this evening, which was a huge plus.
The real crunch point will be when he comes off all the drugs but for now, he's happy and munching with his friends. If he nosedives with no medical support, I'll know I've tried everything in my power to make him better and it's just his time. After losing little Bert in March to colic, I SO don't want to go through it again, but that's all part of the bargain you make when you have them, I guess ...