If the agent placed the deposit with the TDS on your behalf, you should have had a letter (or an e-mail) from the TDS confirming that they have received the deposit and giving you a user ID number. If you have not received this, the likelihood is that that the agent is holding the deposit and has not put it with the TDS.
The very fact that the tenants are in place, and have been introduced to you by them, would probably be enough for them to be able to claim their commission. Did you negotiaite a commission rate with the agent? Do you have anything about the agreed rate of commission in writing?
The fact that the tenancy agreement has not been signed is due to the negligence of the estate agents, so you may be able to make a claim on them for this.