You have an estate agency as your letting agent whose owner is the tenant? Okay..... that's crazy.
Your options in terms of seeking possession are
- "Return" however much this deposit was supposed to be, then serve s21 notice
- Serve the s21 notice as is, and then hope (unlikely since you'll be trying to prove a negative) you can convince the court that a deposit was never received despite what the tenancy agreement says
- If you have lived at the property previously as your main or only home and the tenancy agreement have given notice that such notice ground may be given, serve a section 8 ground 1 notice instead.
Communicate with your letting agent (i.e. address things to the letting agent the company not the owner couple), and seek written response from them on what happened with the deposit that thet tenancy agreement say was taken. I can unfortunately envisage a future where the couple as tenant sue you for the deposit penalty and then you suing the company they own for damages to cover the penalty you paid out to the couple.