One bedroom turned into two bedroom property...

Hi, newb here.
Hoping for a heads-up regarding my small issue.

I'm currently renting a a one bedroom property via a letting agents.
I need to return to the property asap, but unfortunately it's a fixed term contract with no break clause agreed.
I was a bit green and was unaware of this contract.
The tenants have agreed to move in 2 months time, only if I pay them £1600.
This covers the painting they've done, the difference they would have to pay if they moved for the remainder of the year, the time off work, the van required etc, etc.

Ridiculous and disgusting.

I have been a model landlord in that I installed a new £3000 boiler, all
repairs have been done promptly.

I know I really have no leg to stand on.

But, the agent let slip that that 2 women are living in my one bedroom
property, they are not a couple and they have 2 bedrooms. Would this be aSection 18, Ground 17? The property was advertised as a one bedroom property and I'm not happy that they have used the main living room as a bedroom.
I will be seeking advice about this, just thought I'd ask you guys...
Thanks for your help in advance :-)