Tenant has moved out but someone he invited wont leave

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i hope someone can help me...

i had a troublesome tenant that i section 21'd and they moved out 21st September, however they invited someone to live at the house (who didnt go through the checks with the letting agent, but thats a different story) and they wont leave. Their reason for it is that they are disabled and they are waiting for a council house to become available (they applied in April 22) and they wont move in fear of being section 21'd by someone else and it put him back down the queue with the council.

the good news is they are paying the rent, however this has moved to weekly now and its way under value and the tenant who moved is still liable for the house and so is their guarantor, so i have some reassurances.

what are my next steps? is there anything else i can do apart from a CCJ? i just want this person out of my house.