Ok - you need to serve a notice to quit.
Serve a S.8 notice.
You can find a template here:
https://landlords.org.uk/sites/default/files/20P%20Section%208%20Possession%20Notice.pdf
If they do not pay up within 2 weeks then you can seek a Court order for possession.
You can do this online at:
https://www.possessionclaim.gov.uk/pcol/
Or download form N5:
http://s3-eu-west-1.amazonaws.com/hmctsformfinder/n5-eng.pdf
And N119:
http://s3-eu-west-1.amazonaws.com/hmctsformfinder/n119-eng.pdf
The Court will list for hearing and decide whether to give possession.
If the Court does and they refuse to move out, then you can instruct bailiffs.
You can not and must not evict without a Court order and it must be by certified bailiffs.
Can I clarify anything for you about this today please?
If not I would appreciate a 5 star rating for my answer. If you need more help, you can ask follow up questions for free and I would be delighted to assist. Thanks!