The situation is like this: annual gas safety check is due next week. Gas engeneer has been trying to get in touch with the tenants for the last 3 weeks, but they are not replying to letters or answering phone calls. what should the landlord do?

Its in tenants interest to co-operate as they wont be able to stay at the property w/t an annual check being properly done. Perhaps the landlord could call on tenants to explain the importance and urgency.

Perhaps the tenants were away on hols or working when the engineer called. I would persevere with trying to contact tenants asap as you must have this check done. Furthermore, as pointed out above it's in their best interest too. Also, its important to note all the attempts the landlord/agents/gas engineers made to contact the tenants (in case there are any issues in the future).

Phew, managed to speak to tenants direct. Their are happy for me to pass one of their mobile to the gas company to agree mutually convenient appt. Fingers crossed, safety check will be done by the end of this week!