My housemate is moving out, so what do I need to do now?
If you are on a joint tenancy, you're all responsible for paying the rent on the whole property so you're also all responsible for finding a new housemate.
When you have found someone to move in, your landlord will want to put your new housemate on to the tenancy agreement. This is one of the few things they can still charge a fee for. They can charge a flat £50 to change a name on a contract (or make other changes to it) – they can charge more but only if they can prove their costs.
You may be able to negotiate on the fee; the alternative for the landlord would be for everyone to move out, meaning they'd need to find a new set of tenants with the extra costs that would entail.