I'm afraid that you're in a very difficult position. The Courts are very strict about this and if they apply for the monies then they will probably get judgement in their favour years of mismanagement of tenant deposits, is stems from years of deposit mismanagement by landlords.
If they still occupy the property then you can place it in a scheme now, but I'm afraid this is still unlikely to receive much sympathy from the Court. It's not an adequate defence to plead ignorance, I'm sorry to say.
I think you are looking at attempting to reach a settlement with them for their confirmation that they will not make the application at Court. There is a bit of an administrative burden in making a claim (for both them & you defending it) so they might be willing to listen to you. At the very least you should offer to repay the whole of their deposits to them immediately and perhaps some more besides depending on how likely they are to issue a claim.
Information for future reference on tenancy deposit schemes here:-
Information on tenancy responsibilities of a landlord here for future reference:-
Sorry it could not be better news.
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