Jon: then you're misinformed, and placing yourself at risk.
If you give notice to enter and the tenants consent, fine.
If you give notice to enter, and the tenants don't reply... questionable. If they've always previously agreed, you may have grounds that this suggests implied permission to enter (especially if it's in the contract they signed, which may demonstrate assent even though it's not enforceable), but it's not set in stone and most decent landlords would make alternative attempts to contact the tenant. The tenants can't be one missed email/letter/voicemail from you entering the property without their knowledge or consent.
If you give notice to enter, and the tenant says: not on that day, or not unless I can be there, then you can't enter and need to find a compromise. To just overrule the tenant's stated wishes would be very unwise, even for a gas safety check. Again: laws and covenants can contradict each other (in this case you're legally obliged to do the gas safety check, but legally unable to force access), so it's about finding a workable solution.
If you give notice to enter and the clients continually refuse or obstruct, you need to take further legal steps. This still doesn't mean you can just let yourself in. Maybe, if it came to court, you wouldn't be prosecuted, but that's not a given.
Where there are contradictory covenants, you can't just pick the one which suits you. You have to see how the law is interpreted; currently, it's considered that entering against a tenant's wishes is a serious mistake, and that the covenant you quoted is not reasonable defense in a non-emergency situation. Your opinion that nobody would uphold a civil complaint about you entering without permission, because it's for a gas safety check, does not fully reflect the legal reality. The HSE gives a suggested course of action for uncooperative tenants - but it doesn't say hey, they're being silly, of course you can let yourself in.
Yes - tenants who won't allow gas inspections at all are being very difficult and acting against their own interests. If all else fails, a landlord could take legal measures to get access (an injunction, I think? or a warrant of entry, if it's a council property), or terminate the contract at the soonest legal point, or sue the client for damages resulting from lack of access. All liability for gas-related injuries, even death, would arguably lie with the tenant, if the landlord can prove they've taken all possible reasonable steps to inform the tenant of how vital the gas safety inspection is, and repeatedly tried to find an acceptable course of access, and probably co-opted neutral third party help (the gas company?) to tell the client this really is vital.
But none of that means the landlord can just enter, even if they think the tenant's being ridiculous/mad/difficult. Your opinion on their behaviour has no bearing on the legal situation. If you just google 'landlord access for gas safety check' you'll see so many landlord-oriented sites and legal blogs describing how complicated and frustrating this situation can be when a landlord has a completely uncommunicative or hostile tenant.
A tenant who will permit access under certain conditions is not uncommunicative or hostile, however irritating you find them.
(Apologies for the long post, but access is complicated and there's no way I can say either 'yeah, landlords can give notice and go ahead, no problem!' or 'no, tell your landlord to bugger off, end of!'.)