Write them a polite letterr eminding them that 4 weeks has passed, and the deposit is now due back and they are in breach of the rules of he scheme.
Ask to see the tenancy check-out document, if you havent already, sometimes they use "damage" or cleanng costs as an excuse to withold money. If the checkout is clear, they have no excuse.
Give them a date by which you expect the full amount, and state that you have completed the initial papaperwork to commence proceedings for the full amount in the smal claims court, and that if you have not received it by or on that date, you will be initiating a claim for the amount plus costs.
That should do the trick.