Tenants’ giving late notice within fixed term tenancy?

Hi, my property has been let for the last 3 years to a couple of otherwise very good tenants, on a fixed-term contract. I offered them another year’s fixed term but they didn’t get back to me (first red flag) and they’ve now contacted me saying that they’re giving their notice to move out on the day the fixed term ends.

Their contract states they need to give one calendar month’s notice, and they’re under that by a week (i.e. a month’s notice would have been a week ago). I pointed out that they’d be in breach of contract as it clearly states a month’s notice but they’ve just replied that they have been told by Citizens Advice that it’s unenforceable/unreasonable.

This hasn’t happened to me before so I’d appreciate any advice on how to proceed – am I within my rights to claim the month’s rent from them after the fixed term contract ends as they didn’t give me a month’s notice? Keep applying pressure? Or is it a battle I’m not going to win?

Thanks,

James