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So essence, the tenant is complaining about the condition of the property and refusing access to do what needs doing?
What is the reason that she will not allow you in until the 15th.
Why should that be holding up the payment of rent? It shouldn’t.
When you talk about the previous letting manager allowing the tenant to move in with a carbon monoxide alarm, do you mean without? Has that now been remedied?
If it hasn’t, I think you could reasonably justify it being an emergency provided you went into the property when the tenant was not in occupation because the reality is there is very that she could do about that.
Moving into do decoration however is a different thing altogether and that would certainly need more than 24 hrs notice.