Go back round and knock as i bet he aint told tenants as then they will be suspicious of him afterwards. He would never ring tenants saying dont let gas man in as it would look suspicious. tell tenant theres been a recall with make of boiler and for safety reasons your fitting a brand new one there and then, act normal even ask for a cuppa, keep them talking away with you then remove and say you will be back in 30 mins and NEVER go back! and i guarantee they wont even question it as the end of the day they aint paying. They will never ring you as its landlords resposibility to maintain property, especially a boiler in this weather. Promise it will work mate, but remove and cap safely at least.
Keep everyone posted with outcome mate.