In preparation for Transco moving my existing gas supply, I will be
fitting a new outside cavity mounted meter box.

Inside the new box, there is a label which states that a 0.1mm 700mm
wide plastic membrane must be fitted behind the box. While the fitting
of the membrane should present no real problems, my question is what
is the correct type of membrane to use?

Asking this question at my local builders merchants, one said use an
offcut of ordinary damp proof membrane, while another said use some
clear plastic sheeting.

Can anyone advise what would be the correct membrane to use, please?

Thanks in advance, Terry.