We went through a TDS arbitration too! It is really easy to raise a complaint. Just go to this link:
https://www.tenancydepositscheme.com/ch ... spute.html
Put in your Tenant Deposit Certificate Code and then you go from there. You input your evidence (you have a deadline to do so) and then the landlord answers with their own evidence. After that, there is a certain time period where you wait (no longer than a few weeks) and you get a decision from the arbitrator. When you raise your complaint, you include your bank account details and so if the arbitrator finds in favour of you, the money goes back into your account- easy peasy.
For the record, I also would not recommend Douglas & Gordon in Clapham South. We hardly ever dealt with the same people twice. I think there's a lot of turn over in those places.