I was off-line yesterday when your reply came in.
If you let me have your telephone number I would be happy to call you this morning.
Here is quite a good article on the grounds for giving a section 8 notice:
It’s much easier to read than the section of the act.
Please note that grounds 9-17 are discretionary so you are not guaranteed to get possession unlike grounds 8 where you are BUT there must be 2 months arrears at the time of the hearing and he can reduce the arrears on the day in court and your grounds 8 fails.
You would therefore issue on grounds 8 of their RNs arrears, 10 & 11 if the rent has been spasmodic, and any of the other 4.2 grounds which apply. You can put as many grounds as you like.
You are going to have to have evidence.
Here is the government website on section 21 & section 8 notices including the forms: https://www.gov.uk/evicting-tenants/section-21-and-section-8-notices
I would suggest that you served both notices including a section 21 to end the tenancy at the end of the term but please note the criteria for serving it.
Just because he has mental health issues doesn’t mean he can do as he likes.