I've found the dinner plate.....!

Our aitightness test a week ago had me crawling around in the loft at the wekend with a big torch which revealed the achilles heel of our aitightness strategy - penetrations and unsupported joints!
For the most part the vapour/air membrane is continuous within the building with joints made with tape and silicone which were then mechanically trapped beneath battens or plasterboard.
The exception to this is the loft space where the membrane was simply stapled to the underside of the rafters, between which insulation was previously fitted. This clearly makes the membrane vulnerable to gravity (!) and any air pressure difference between outside and inside the house with the ultimate risk of the membrane pulling away from the rafters - which has started to happen in one place. I have also spotted two un-taped long joints with resulting gaping holes exposing the insulation, and an unsealed soil pipe penetration. These gaps must easily account for at least half of our dinner plate size hole given that the soil vent pipe ducts were open in the bathrooms during the airtightness testing.
The ultimate solution is probably to bring down the membrane from the rafters to the loft floor and place the insulation from between the rafters on top of it, thus creating a 'cold' loft. It goes wihout saying that the penetrations need sorting out also!