Dad Ended Rolling Tenancy And Landlord Is Threatening To Sue.

My dad has lived in the same maisonette for 12 years. He signed one contract when he moved in in 2012 and none since. The property is very outdated with no double glazing, old rickety mouldy windows and an outdated heating system that means he needs the heating on constantly in winter and it often breaks down.
He has had to replaced all the radiators himself a few years ago as they started to leak everywhere and the landlord chose to do nothing. It was emergency work and the heating engineer said they were so old and rusted that they had to be replaced and could not be repaired. So he paid for all this himself. He has agreed to several rent increases all with the property never improving. I used to live here too and in the time I was there we repainted all the walls, installed a power shower as there was a basic handheld shower that was so weak, replaced the doors as they were old and cardboard and falling apart. In all this time my Dad has been very patient and kind and paid his rent in time without fail.
At one point, the landlord begged dad to lie to his broker about how much rent he paid as he was remortgaging the property. My dad did just that. I’m not sure if that works against dad or not ?
Dad is moving to my town to be near to me and my daughter. He will pay the same amount of rent for a much nicer apartment which is clean and new and will be a luxury for him. Anyways landlord went mad when dad gave notice (like 6 weeks) and said dad was obliged to give 6 months notice. We don’t have the contract it was so long ago. He has threatened legal action and I (qualified U.K. solicitor) will defend dad if this happens. Dad is a pensioner and is very worried and stressed at what should be a happy time as he is excited to move close to us to a much nicer area. Do we have something to worry about?
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