How fast is the meter going round when only valves 1 & 3 are open, I would have thought if it is a drip type leak then you would not notice the meter moving by eye but only over a period of time. Cane you close valve 3 and open valve 2 and then some tap and try and replicate the meter speed to give an idea as to what sort of volume we are talking about. In addition where do you live? There may be a member that lives nearby who could pop round and give a second opinion.