Good news! 
The vet school called again this morning and said Rollo had had a good night and only one poo - NOT diarrhoea?!? but that it might take time for the antibiotic to work out of his system (they're waiting for diarrhoea to return).
However, they phoned again a few moments ago with an update. His poo is still fine and once they have 3 samples to test, he'll most likely be discharged on Saturday morning.
They've found two kinds of problems with his ears, meanwhile - a fungus infection and some small cells infection which they said was fairly common in breeds that have fold over ears - rather than erect ones. He needs me to put in drops for 15 days, post discharge. Last time I tried to put ear drops in him, he growled at me as he clearly didn't like it and I couldn't carry on with that treatment. Hope he'll be OK this time.
They also think he maybe could/should return to his original Royal Canin Golden Retriever Junior 29 food - as the current one (Sensitivity Duck and Potato) is too high in fat. hope this won't then start off further diarrhoea...
Will he be emotionally OK staying in the vet hospital? I think it'll be much more like being in an upmarket kennels but he's only ever been used to my home and the trainer's home and loads of regular contact with people and two proper walks a day. They do walk them short walks on lead and they've given him a ball to play with - but he's used to having tons of toys to chew and play with and cuddles etc from me. Hope he's all right. They said he was fine and happy to greet and play with staff.
Anyway, I guess if I'd dared to take him off antibiotics myself, I could've found he'd no diarrhoea and saved myself £1,238.00! They might not have picked up the ear problem at the vet however. Also, until they get 3 poo samples, they can't say for sure if there's been anything nasty in his system that's still lurking.
Meanwhile, still feeling pulled between Rollo's need and DCs needs. DCs are happy he's away again and don't really want him to come home. The last evening we walked him together, I had this flash of insight that if I didn't have the DCs - it'd be lovely to be doing at least one proper country walk off lead per day, after work, getting fresh air and exercise. But with 2 grumpy, fighting, whining DCs in tow - it's a nightmare. DS1 would happily remain behind at home - but he's the one I wouldn't trust to be left at all. DS2 would be fine at home as he's trustworthy - but he gets phobically terrified if I go out even for a few minutes to the local shop 4 mins away and swears he'll NEVER let me go walking the dog alone for more than 15 mins and MUST come with me, although he HATES the walks!!!! Oh vey!!!!!!