As the parent of a student just finished second year, and renting a new place as part of placement year, my advise is photograph everything and send in emails about all items that are broken, stained, dirty etc. Even if they don't get answered, you have the evidence that you sent them.
He did this, the only things they didn't photograph was the base of mattresses, as never turned over, and guess what, the agents are wanting to retain half of deposit for new mattresses on basis of stained bottom.
Interestingly the inventory included photos of mattress tops, showing stains, but not bottoms, so I would think the agents would be hard pressed to argue that the bottoms were unstained and pristine a year ago, either to the deposit scheme resolution people or a court of law!!
Mind you, they are also trying to claim for oven clean, despite my DS sending in a photo of the filthiest oven on this planet a year ago and a pristine oven when they moved out. The words "trying it on" come to mind. Maybe they think some students wont fight them, but DS and his housemates certainly will. The lead tenant is a slightly older girl, and not the sort to be messed with at all. Also has enough money to take it all the way to court, as do we.
DS will be taking more photos of his new place this week, including both sides of mattress. He is one of 4 in a 5 bed house, the mattress in the spare room was disgusting when we moved him in 3 weeks ago, so again, I have told him to photograph that, and go through inventory in detail with other 3 ( all placement students like him!!), checking what damage or stains are noted and making agent aware of anything missed eg mattresses, along with photos.