I am a ll so sorry to hear about this, my worst nightmare tbh.
They need not have given notice but it is nice to, so plus pt for them there.
Sadly if they stay 1 extra day over 6 mth ast they trigger a periodic contract by default & they are then on 2 mths notice from you, 1 mth for them.
I’d go & talk to them, remind them of their obligations & what the effects of a ccj are. Take a witness or video the meet as they may pull a dirty & accuse you of harrassment.
Then go legal, serve notice & go after arrears, do they have any money or a guarantor?
Good luck, don’t let them get away with it.