I agree with the solution offered by 'thesaint' but totally understand your concerns about the behaviour of your former agent.
I suggest you write a 'letter before action' to the agent, giving them 14 days to refund your £35 and provide the information you want from their files. Make it clear that you will be taking the matter to the court if they don't comply - not only for the £35, but also for breach of contract (an implied factor in every contract is that the parties will act within the law) and some form of damages in respect of their actions leaving you open to legal action from your tenant.
None of the above would be worth doing if they call your bluff, damages would be minimal and would the paperwork and an afternoon in court really be worth it for £35?
If you like the information/help provided - Gizza Job
(People under my age won't understand the significance of those last 2 words)