You should have had confirmation who it was being held with shortly after you paid the deposit. I would just remind the land lord/letting agent, if you get no joy go straight to the deposit holding company. Just be careful and don’t just roll over if they try and deduct anything.
Worked for me when they tried to keep my whole deposit because of “rent arrears and cleaning costs” just submitted my bank statements showing the rent had been paid on time every month and the agents were unable to provide any proof of the supposed cost of cleaning so I got all my money back within a few days of sending this to the desposit company.