Tightness test not legally required for landlord cert. You can buy a u gauge to test gas but do you know about letby , temperature stabilisation, and the permitted losses depending upon type of meter .do you have access to a fga and are familiar with the co/ co2 ratios to say that it is safe. Are you aware of bs 5440 part 1 and 2. Maybe all the above have a little bit to do with being competent