oh gosh, lots of new replies, thanks very much to you all for taking the time to reply 
betelguese, he is away until Monday now, but will see the accom office then I hope, & see what is available. I will suggest to him that he looks out for ads in house shares but as he signed up to the year in uni accom I imagine he is tied to that. His current accommodation is provided by the university so I don't imagine the landlord issues you mention apply here. As he's only halfway through the first term (& other universities are not nearly as full-on as Oxford!) he really doesn't have a lot of work, & as far as I can tell his studies aren't affected - just his sleep, sometimes...
philmassive, that is v useful information, thank you - I will mention going to the union advice place, & suggest that he hints that he might have to leave if he's getting no joy otherwise.
boffinmum, I hope he will have heard from his personal tutor by Monday, & have a better idea what he can do. Sheffield is very highly rated for student satisfaction in general so we can test that out by how they handle this!
sashh, he is reluctant to speak to the subwarden in case it causes more aggravation. I'm hoping the personal tutor/accom office between them can sort this out
campergirls, we'll see what happens next week; I'll try to nudge him towards student support if necessary.
He's OK today anyway, I had a text this morning from Manchester & he should have arrived at Sussex by now; he's with 2 good friends & will have a great weekend & I hope he'll be sorted out by this time next week.
I'm very grateful for all the advice given on the thread. MN is so useful at times like this 