I should add that my contract is an AST (Assured Shorthold Tenancy) if that helps?

This is the most common type of tenancy, ours is exactly the same. We've been in the property for much longer than our fixed term and if our landlord tried this I would report him to the DPS if he withheld any part of our deposit.

Did you receive confirmation that your deposit was placed in the Deposit Protection Scheme? If he hasn't done this you would already have grounds to go after him as this is now a legal requirement. If a landlord withholds a deposit unreasonably DPS can investigate and can award up to 3x your deposit back. I would kindly remind your landlord of this and let him know that you will not let the issue go.

More information from the government site: https://www.gov.uk/deposit-protection-schemes-and-landlords