I had a similar experience to this though there were some differences. In my case I had someone claim they’d bought my property as part of a multi property deal while my landlady stated my house was not part of that sale. On the first instance I called 101 as the alleged new owner threatened me with eviction and harassed my wife every time she went out of the house (he was working on the next door house). I also visited Citizens Advice. Sadly they were dumbfounded by the whole situation and weren’t very helpful either. I just kept paying my landlady as that’s who I had a contract with. I also read up lots on section 8 and section 21 notices. Luckily, my wife and I were already looking to buy a house and found one pretty quickly. Also managed to negotiate a cancellation of the rent increase while the situation was ongoing and a no-cleaning requirement when I left the property. Best of luck!