Thanks for your message, I appreciate it is important to you.
You have a legal interest in the property. If the ownership of the property changes, that interest is not destroyed. This means your tenancy will persist through and after the property sale. You still benefit from all your tenancy rights as described by the law. None of them are limited, altered or restricted by the change of ownership.
The landlord, who purchased the property is forced to accept you as a sitting tenant. If an agreement is already in place, with the old landlord, then this is the agreement that will stay in place unless you come to a new agreement with the new landlord. There is no obligation to agree to a new agreement, but you can if you want to as long as all the parties are in agreement.
It sounds like you are being coerced into signing a new agreement for 1 year which you do not want to do. You do not have to do this, if you do not want and if your circumstances have changed. If your old tenancy has expired then the new landlord can still use eviction procedures against you, but they need to follow the established rules.
You can discuss with the new owner a new short let agreement, but he does not have to accept it. If no agreement is reached, then your old agreement prevails.