Student AST issues

Other

I am a landlord and currently studying for the level 3 award, so have some knowledge, but I have a family member who is a student in a situation I'm struggling to advise her about. I'm hoping you can help.
She, and 3 friends have an AST which started 1 Aug, but with a retainer period to 31 Aug, during which time they could move in their possessions, but not move in, and only pay 50%.
The house was empty at the start of August,but when taking belongings there towards the end of August they found the landlord and his family were living there. Can he do this? Does the 'exclusive possession' still apply in a retainer period? Could they claim breach of the lease and ask for some of the retainer fees back as 'damages'?
The agreed rent also includes utilities, but the landlord is now saying that the heating is programmed to come on between 6:30 and 8am and 6 to 10pm and they are not permitted to change this. With the Covid situation it's likely that they will need to spend more time than 'normal' in the house, and one of the reasons that they went for a bill inclusive rent. Will this become an argument about what is "fair and reasonable"? I feel they should clarify this now, to avoid issues later.
I appreciate any help.
Thanks in anticipation.