Sec 21 issued, 1 tenant leaves, 1 stays?

Hi, I have been a landlord for 20 years and am now selling all my properties, one a year, as I’ve had enough of the way things are going against the private landlord.

I have done evictions in the past but have now come up against a situation new to me, so would appreciate advice from those of you with experience of the following.

I have issued a section 21 to a couple who have been with me for several years. The notice requires them to leave on the 20th of this month.

They know it is because I’m selling as I told them 18 months ago, hoping they would leave earlier and save me the trouble of eviction.

One of the tenants has found a place and is moving out on the 15th, but says the partner is staying on as advised by the council.

My question is: As one person has left the property, does the tenancy end automatically when the S21 notice is up on the 20th?

Can I tell the other tenant to be out as I’m coming to change the locks, no matter what the council says, or do I have to continue through the court system and bailiffs?

Thanks for any lawful advice,

R