I KNOW I've seen them advertised somewhere obvious but I must be having a senior moment

Has anyone got a link?

http://www.nfauto.co.uk/cbs_products5.htm

"Cockpit Ventilator" is what I think you mean by helicopter vent.

Yep, thats the one
Ta very much

Helicopter vent made me think of the ones that you used to see on bus and van roofs that spun round as the vehicle moved

Made me think of the flipping huge slatted grills on the engine bays of turbine heli's