My flatmate refuses to leave. Can I make them go?Beta*

It depends on who rents from whom. If you are both joint tenants on the same agreement, neither of you can make the other leave. Be aware that if one joint tenant gives notice to the landlord, it ends the tenancy for everyone. If they are your lodger or subtenant, you are effectively their landlord and must follow the right steps: usually reasonable notice for a lodger, and a court order for a subtenant. Changing the locks or removing someone's things without following the process can be an illegal eviction. Get advice from Shelter or Citizens Advice first.

Sources: Shelter: joint tenanciesShelter: lodgers