I am assuming that you have come to the end of the term or you have given the notice required under the terms of the tenancy agreement.
As this is a corporate light, then none of the statutory safeguards apply.
You cannot beat the landlord with a stick and he will pay deposit back to you, you have no alternative but to take him to the Small Claims Court. I assume that you have an address for him and if not, you have one for the agent.
You can issue Small Claims Court proceedings at his last known address or care of the agent quite easily here www.moneyclaim.gov.uk
Before doing that, right to the landlord and tell him that he has until 4 PM on Friday, 15 June, to return your deposit failing which you will issue Small Claims Court proceedings without further notice.
There is no point in all honesty in messing about exchanging copious correspondence.
Can I clarify anything else for you?
I am happy to answer any specific points arising from this.
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If you still need any point clarifying, I will still reply because the thread does not close.