rent arrears and boiler problems

Rent Arrears

1. Tenant, upon moving in on 1.9.13 paid only half of his deposit & to date still not received remainder.

2. Despite lease agreement requiring standing order, one has not been set up & tenant paying only when requested, making excuses for delay in payment.

3. Tenant reported boiler problem on 16.11.13 and plumber attended property twice over a period of time.
4. Tenant still complaining he can only get hot water when he puts the heating on.

5. Tenant now refuses admittance by plumber and states that he will call in British Gas.

6. Tenant then reporting that the boiler is unsafe and about to blow up.

7. Giving Tenant notice by letter dated 23.12.13 to vacate by 28th February 2014 , being 6 month lease – AST)

Questions:

Can tenant refuse to pay rent due to, so say, faulty boiler?

Can I use the half of the deposit as unpaid rent? If so, how do I go about doing this; i.e. removing it from the Rent Deposit Scheme?
Can tenant refuse my plumber entry?

Can tenant call in British Gas without my permission?

Can tenant refuse myself, as landlord inspecting boiler if I give 24 hours’ notice? (it was over a week ago he said it was dangerous)?

Can tenant delay this month’s payment (due on 1.1.14) pending receipt of a British Gas report (however, British Gas state they have not received a request for a visit)?

I have sent a letter dated 13th January 2014 requesting payment (remainder of deposit and 1 month’s rent) by 17th January 2014 – How do I follow this up now?