The Deposit Protection Service (DPS) could be managed as a separate bank account. Any payments made to the Deposit Protection Service could then be tagged as a bank transfer to the DPS holding account. That way the collective balance will show as an asset on your balance sheet, i.e. at some point it will be repaid by the DPS when the tenancy comes to a close.
This asset should then be offset against a liability that would represent any bonds paid by the tenant to you as the agent. Again this could be defined in another bank account called something like “Client Deposits”.
The above should cover the basics, but I would recommend also getting a second opinion from your accountant on this.
EDIT:
There’s also a useful explanation here that works along the same lines:
More general info on Tenancy deposit protection.