Tenant/landlord dispute

Questions:

• I am a tenant and wish to sub-let. What is the law regarding sub-letting?
• In my tenancy agreement it says my landlord has to carry out repairs and he hasn't. What can I do?
• My landlord has changed the locks and I can't get in, can I break in?
• My tenant has not paid their rent, what can I do?
• What can I do about squatters in my property?
• What is the correct procedure a landlord should follow to legally evict a tenant?