Tenants planning to do a runner

Rent Arrears

Hi I have an HMO property with a few tenants who are on benefits. Tenants never pay the difference between LHA and rent required in order to run the place and do maintenance.
I have three such tenants, I have served them with notice Section 21B. One will shortly be in Court for burglary and drug dealing, he has damaged his door and has his own lock in place,I have no key in the case of an emergency. He has told me he will be leaving shortly.
Another tenant was in care and being guided by her Leaving Care Worker. She was eighteen in May and instead of handing her over to SHDC she applied and got her rent paid direct to the tune of £1050. Lincs County Council paid me £500 of her debt and she has to date paid me £100.
She is blatantly planning to do a runner as she just told my friend. Her leaving Care Workers accept that they slipped up in not ensuring I got the rent direct. What would my best course of action be please? She going to go and live in Lincoln near the prison, either way she is planning to go with the 'burglar' She has just received some compensation so is in a position to give him money so she is quite popular at the moment. This of course doesn't matter, however I would like to get my £450.