Any complaint to the landlord, which might have any affect on paying the agreed rent, should be put in writing. Then if they look to take any action, you can document that you have advised the landlord of the issues.
Shelter or Citizens Advice should be able to advise further.
http://england.shelter.org.uk/get_advice/complaints_and_legal_action/complaints/complaints_about_private_landlords
If the gas boiler did not have a certificate,then the letting was probably not legal. I thought that it was illegal to let a property without the annual certificate ?