Firstly the tenant should have signed on the AST that the landlord can enter the property for essential maintenance and repairs provided 24 hours notice is given
It doesn't matter one iota what they signed, the law overrides the tenancy agreement and that law states you cannot enter without the tenant's permission except in an emergency. Gas safety certificate, while important, is not an emergency.
You should make sure you've taken all reasonable steps to arrange the safety check such as letters sent by recorded post, phone calls (use your mobile and screenshot the time/date/duration of the call), and emails or text messages. That's your back covered, you've tried and can prove you've tried, any consequences are then on the tenant's head.
Do not force entry and do not cap off the gas, both of these will only exacerbate the situation and you would be breaking the law.
Seek legal advice about serving notice to make sure it's all done above board and you meet all your requirements.
You said there have been no replies, are you absolutely sure they're getting the messages?