Firstly, the Assured shorthold tenancy is not applicable to the let. Your tenant is a lodger and as such is an excluded occupant, as you rightly said. As this is a lodger arrangement the deposit does not need to be registered with a tenancy deposit scheme, your lodger is using this to intimidate you.
You can evict the lodger once you have been given reasonable notice. You may need to give 2 months notice in line with the written agreement.
You do not need a court order to evict you.
If you have a fixed-term agreement, you can only evict the lodger during the fixed term, unless there's a break clause in your agreement.
You can serve notice on the tenant, use the deposit against any arrears and Sue the tenant in the small claims court for the remaining arrears.
The lodger cannot sue you for 3 x deposit.
You can report his intimidating behaviour to police,it is harassment.
Further info, if the lodger won't move out, after you have given notice and the period expires, you can change the locks while the lodger is out and remove the lodger's belongings from the property and place them outside.
You do not need to go to court to evict the lodger.
Any questions or queries? .
Any damage caused is criminal damage. That should be reported to the police.
Do you share any living space with the 'occupier'.? I.e lounge, kitchen, bathroom. If not then the occupier may have basic rights and you will have to serve the proper notice, likely to be two months under the AST you entered into. When was the tenancy granted and for how long?
Break clause has nothing to do with non payment. It's a clause allowing early termination.
The 21 day clause is probably the forfeiture clause but this is fairly toothless in residential Tenancies. A small payment towards rent could defeat the provision.
This is a simple question and answer service. You have a difficult situation. It will depend upon the living arrangements as to whether the occupier is excluded or has basic rights. The tenancy agreement which you and the tenant completed has to be considered in light of the status of the tenant. If the tenant has basic rights then proper notice must be served and you will need a possession order from the court.
Non payment of Rent is a ground to apply for possession and depending on the type of notice served there is a set notice period, which can be as little as two weeks. Damage to property is another ground.
I would advise going to a local firm which has a property litigator. Have a ring round, sometimes you can get an initial meeting free if charge. Ensure to have at hand details of arrears, any written demands, and written correspondence between you and the occupier, details of intimidating behaviour (what when), details of any damage to property, what / when. The litigator can assess the appropriate notice and serve.
Any questions or queries?
Happy to assist further.