firstly, have you registered the tenancy with PRTB?
you could sue in D/Court, but do you know if the tenant(s) or at lteast the one that signed the lease has money/assets? Otherwise, you will get a judgement but if there is no money to collect, what good is that?
You have been had. it happens the best of us. learn for the future. move on.
tidy up, advertise again and let, but this time, with one month's rent as deposit and one month's rent in advance. that way, if worst comes to worst, you at least have some few quid.
Alternatively, despite what some posters feel, let to RA recipient, with weekly rental payment made by State direct to your bank account. You are guaranteed the rent and the tenant may pay alittle extra to top up.