It will all depend on whether your replacement housemate signed a new contract and your landlord took you out of the contract and gave you your deposit back. If this is the case, then you don’t have to worry, you are not responsible for paying.
If no new contract was signed, or you had an assignment notice saying you are still liable if your replacement doesn’t pay, then the landlord can still chase you for the rent.
You can take your replacement tenant to small claims court if you have had to pay and you think they owe you money.
LUU Advice can help you by looking over your contracts and agreements to check whether you are liable to pay.