Complete deposit retention. Is it fair?

BitterAndTwisted

A landlord is entitled to compensation from the tenant for dirt or damage. There is absolutely no obligation, legal or otherwise, for the landlord to actually have it carried out. Please don't try to muddy the waters and mislead the OP by telling them to insist on seeing invoices!

serious_saver

Did your son sign and return a copy of the inventory. If he did not then they have no proof that he agreed with the inventory. This a particularly the case as the only inventory they have has a different date on it to his contract.

He needs to check that the deposit was properly registered. There are search tools for each of the 3 schemes (links below) Even if the agent won't provide a reference number they should be able to tell him which scheme it is registered with and you can fill in the gaps.
http://mydeposits.co.uk/tenants/get-started/check-your-deposit
https://www.depositprotection.com/is-my-deposit-protected.aspx
http://www.thedisputeservice.co.uk/is-my-deposit-registered.html

If the agent is trying to claim the full amount of the deposit then you son should definitely go to arbitration. After all he has nothing to lose.

Once the process is started the agent will be asked to submit proof and you son will be asked to do the same. The worst that can happen is the arbitrators will find in favour of the agent. However, even if your son only gets some of the money back it will be better than nothing.

These schemes are designed for situations like this, where there is some sort of complication, so both the landlord and tenant know they are getting a fair deal.

nigem

We have just had the adjudication summary from TDS. Because there was no check out inventory from the 1st 'set' of tenants OR a check in from the 2nd 'set', they have found in favour of the tenants. They returned all the deposit minus the sum for replacing the locks (£75) as keys werent returned in time.
I do feel for the LL and hope that they sack the Agents!
Thank you everyone for your help.