Advise please?

Other

I have tenants who are threatening to take me to court for not depositing their bond in a deposit scheme. I didn't know about this scheme until recently. They have left the property suddenly, after being burgled. They were in the property 4 months into a 6 month assured shorthold tenancy agreement. They paid rent for the 4 months but gave no notice of leaving - they left on the 5th December but have paid up to the 27th December. The agreement started on 27th August when they moved in. I have returned all of their bond to them. They say they will not take me to court if I sign or email them to say their tenancy agreement is finished. They have not yet cashed the cheque for their bond. I have checked their utility bills and they are paid up to 4 days before they moved out.
They want me to sign to say their tenancy ended on the 5th December - the day they moved out.
Should I sign to say their tenancy is finished - releasing them from any further liability? If so, what date should I say it has finished - the 5th, when they moved out or the 27th when the rent runs out? If I sign and they decide to take me to court after I have signed, I assume I will not be in a position to claim back the rent from the other 2 months.
Any advice would be most appreciated.