Yet again - absolute crap from MNers regarding renting issues.
A LL only has the right to enter a property against the tenant’s wishes in an emergency - fire, flood, threat to life.
A GSC, as important as it is, is not an emergency.
Doesn’t matter what clauses are on a tenancy agreement, the tenant retains the right to decide who comes in and when. A LL can send as many “24 hour notices” as he/she likes, if the tenant says no, that’s it.
OP...if you want to gain access for the GSC, you need a judge to sign off on it. Councils and HA’s regularly go to court to get warrants allowing access to properties for gas safety - if it were as easy as sending notice then coming in, they’d do that. They don’t, because they can’t.
My advjce would be send a recorded letter requesting access for the gas engineer & explaining why it’s in the interests of their safety that this is done. Wait another couple of weeks and send another. Up to you if you then approach the court for a warrant of entry or wait till May.
You do know that the tenants may not be planning to leave at the expiration of the notice, right? There!s nothing in law that says they have to, so be prepared for that.