Ok thank you.
If he continues to not pay rent, you can serve him a notice of eviction for possession of the property for rent arrears. You would need to give him 6 months notice in line with the Coronavirus Act 2020 if he has less than 6 months rent arrears. If it is more than 6 months rear arrears you can give 4 weeks notice. But he must be at least 2 months in arrears for you to serve this notice.
Technically, he could refuse to leave on receipt of the notice, so in that case you will then have to take him to court to obtain a court order. You will need to serve him a section 8 notice which I have attached.
If your daughter and husband plan to purchase the property, then the tenant will still have rights. This means that regardless of the sale, they will still have tenant rights to occupy the property even if there will be a new landlord. The tenant has a legal interest in the property so even if the ownership of the property changes, that interest is not destroyed. This means the tenancy will persist through and after the property sale. They will still benefit from all their tenancy rights as described by the law. None of them are limited, altered or restricted by the change of ownership. The correct legal procedures will still need to be carried out to get the tenant evicted.
If you are about to have repair work done to some of the windows, I would suggest using the opportunity to speak with the tenant to advise what you intend to do and just see how much of a challenge it could potentially be. It would be wise to do this before serving any formal letters/notices just so the tenant does not become difficult. It could be the tenant is unemployed or struggling to pay the rent so they are a bit embarrassed about it hence ignoring your calls. Find out what is occurring and then assess your position then.
Does this help clarify the situation?