Remember that burning gas gives you Co2 which helps to give you more leaf
and better flower on plants when present in Daytime,
If your glass overlaps sheet by sheet then you will have a certain amount of
air movement through the joints, and I have yet to see a glasshouse that is
air tight.
If you are thinking of fitting Automatic vents the Argos does a good one and
it's the best price I know, but only in the Summer catalogue. (I have one
still working after 9 years, most last 5 to 6 yrs).
If you are looking for a gas heater then you might try
http://www.cmsgardens.co.uk/heaters.htm

Got my last heater from them, good price and good service

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk