It would be a good idea to find out whose responsibility it was to ensure that the deposit was dealt with properly (ie put into a TDS).
You are the landlord so a good chance it may have been essentially yours but, if so, was there any (proper and legally enforceable) passing of the responsibility to the agent to make them liable eg do their terms and conditions in any contract between yourselves confirm they will deal with all this, specifically or within a provision which would cover it?
Out of interest, how did the authorities find out the deposit wasn't in the TDS? Was there a problem with the tenants getting it back from the agents, were they querying the £100 etc...?