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Do they have access to the annex?
is the contract is silent on the point of electricity bills?
The reality is that whoever is on the electricity bill is liable to the electricity company. There is no way around that. The fairness of it will not interest them.
The question is whether or not you can recover from the landlord. He does seem to be offering something which suggests that he does have some responsibility in his mind.
It seems, on the face of it, that it depends what agreement existed between them. If an agreement existed that it would be divided equally then that is the liability of the tenants in the annex. That might be very unfair but contracts are sometimes.
Another option is to try to argue that you didn't ever agree to have included the electricity of the annex. To be wholly honest, that might be extremely complicated and it might be that you don't recover much more than 30% but it is an option available to you. If the contract is completely silent on the point then that would help you.
Can I clarify anything for you?
Oh I see.
He cannot be forced to instal a meter I'm afraid. There is no way around that.
All you can do is try to negotiate over that.