The co-tenant we lived with stopped paying her rent. Naughty girl. Surrey & Hants didn't tell us this and instead allowed interest to accure while the contract ran. Annoying, but no disaster - it's only 4.5%.
I said I'd pay her rent for her, but Surrey & Hants seemed to think 4.5% on £425 for two months equals £250 in interest. They also deducted £120 in fees that weren't in any contract that we signed, amongst many other errors I won't bore you with. I tried to reason but they told me to go away (but in nastier words).
In the end, as they would only return £200 of our deposit, I sued. Upon receiving the claim the landlord's solicitor was incredibly apologetic and agreed they would pay everything back immediately - over £700 without fuss once someone who was a) qualified and b) intelligent got involved. Of course, I still paid the landlord's the co-tentant's outstanding rent.
Actual interest due: £4.55.