Simply say no to the rent drop. If they do no meet their rental payments serve them a notice to rectify, and if they then do no correct the issue then initiate proceedings with your tenancy tribunal.
Do they have a fixed term lease or a periodic lease. If they have a periodic lease serve them a notice to vacate - from memory they will have about 60 days to leave. If they don't leave then again initiate proceedings with your tenancy tribunal.
You shouldn't need a solicitor to goto a tenancy tribunal as it should be something you can handle.
