Something similar happened to DDs friend. She was charged £40 for a 'wipe down' of her room. Multiply that by several hundred students and someone is making some money.
Unusually DD has allowed me to deal directly with the agency. I politely asked for a check out time as I wanted to be there. As a landlord myself I would ensure she left the property in good condition, so that their whole deposit could be returned. I reminded them that it was filthy when they moved in. I got an equally polite reply back, oddly copied to all the staff in the agency, welcoming this commitment to leaving the property in good order, which should allow them to return the full deposit.
If they do try to grab £40 from each of them, I foresee a problem. If DD did raise a dispute she would win, but it would take months to get any money back. Because they looked after the flat, the landlord is getting next year is about 40% higher than what DD and her friends paid. It would be wrong for them to be penalised but equally her flatmates probably can't wait six months for their money back.
I think my email was enough. If not, when she raises a dispute she will inform them that she also intends to issue a complaint to ARLA, the University, the Council, and just about anyone she can think of. It wont be worth them dealing with the fall out. I wrote a similar letter for an overseas mum who was having problems with a high end London lettings agency (I think they assumed she was moving abroad so would just give up) and it worked like a dream.
Why does my daughter say I can be embarrassing?
We did think of leaving some Cilit Bang black mould remover for the incoming tenants with a note suggesting they would need it. A contrast to DD finding the previous tenants' drug stash when she moved some furniture to vacuum.
The new flat is much nicer, rented directly from an apparently good landlord, though it costs more. Perhaps one day the old flat will be seen through rose coloured glasses. It was a happy year.