Company let put additional men in property

General Discussion

We agreed a company let for 4 people for 6 months on behalf of a Landlord.
Both sides seemed very happy at the outset but it quickly went pear shaped!
The Landlord was made aware that the tenants had at least 6 men in the property within a month of the start of the tenancy. We didn't agree that it was a problem, they were only staying 2 or 3 nights a week other nights there was only 3 men there, the property is more than adequate for these additional, occasional people. We had also achieved a premium rent of £150pcm over what he could normally expect.
The Landlord threatened that he wanted them out and was going to change the locks, then he visited them on several occasions and agreed an additional £200pcm.
In my opinion the Landlord has harassed the men by sitting outside the property in his van at 6am and turning up on several occasions without a prior appointment. He also stopped their cleaner going in at the week-end questioned what she was doing there.
The tenants now want out, are they within their rights to give a months notice?