Tenants have failed to register for council tax

General Discussion

My tenants have a 12 month AST which began in August 2013. They are liable for council tax but have not registered and have built up considerable arrears. I have provided them with advice and the appropriate form but they are still refusing to register. The also have rent arrears.
The local council have our name on record as the leaseholder and are threatening us with legal action for unpaid council tax. Bailiffs have already visited the property twice but have now agreed to refer the matter back to the council.
One of the tenants is unemployed and receiving housing benefit, the other two are working.
I have informed the tenants that we can deduct these costs from their deposit at the end of their tenancy.
We do not plan to renew their tenancy when it expires.

Any advice on this would be appreciated.