We are about to let our house and the landlord's gas safety test has
detected a fault. When the system is shut off from the main the
pressure after at meter drops by about 0.01 mB per second. I think
the pressure was around 20 mB. It was a digital meter.

Now, the first puzzle is that BG and another plumber both passed the
pressure test a year ago when the meter was moved and some pipwork
rerouted. Nothing has been done to the gas system since then.

The second puzzle is that he said that when he measured it, the
pressur "held OK for a bit and then started to drop".

The house has a mix of copper and old iron gas pipes, all are capped
except at the boiler. It's a 3 bed end terrace.

Any thoughts?