Normally landlord would not allow sub-letting. There is a big chance your landlords are doing this behind the back of the house owner/estate agency.If it is the case, there is no way they could abtain a court order;furthermore court order costs time and money. It could be lot more than your deposit.
What they are doing is illegal. If you know which estate agency they rent the house from, you could threnten them to grass them. In that way, they will loose their deposit, may be taken to court as well.
Just tell them to give back your deposit, otherwise you will call the police and inform their crime to the estate agency, and tell other tenants what they have done to you. Because they can't evict you, you could make a lot of trouble in the house, you got nothing but time to lose.
Good luck.