Do you know whether the deposit was protected in a recognised deposit scheme?
Were you given those details?
Have you filed a dispute with the deposit scheme people?
Is there a reason why the deposit is not being returned to you?
My suggestion would be to agree to whatever is being put forward by way of returning the deposit to you to get as much from it is you can but only agree under pressure/duress.
Then, you take the landlord to court for the balance of the deposit through the Small Claims Court. There is nothing which stops you going to Small Claims Court after you’ve had part deposit back from the scheme.
Remember that the small claims process can take up to 12 months. There is no way of speeding that up.
The 10 day rule only applies if there is no dispute. If you cannot agree with your Landlord as to how much of your deposit should be withheld then the two parties have the right to go to alternative dispute resolution. This service is provided free by the scheme protecting your deposit.there is no statutory timescale for that.
Or, as already mentioned, you can take the landlord to the Small Claims Court.