Brian Reay wrote:
2) if yes, how big. The figure of 100cm3 was mentioned but our local
hardware stores don't carry stock and the minimum order requriements
mean I'd look online.

Last time I checked (about 7 years) it was 16 sq in, which is about 100cm^2.

The area is the "free flow area" (ie area of holes) not the overall vent
dimension.

OK, any clues on suppliers - i've only had a quick search but it didnt
deliver me anything useful