Don't know if this will be of use to anyone else but I have just been out in rain and gusting winds (25mph avg, gusts to 60mph) to override the thermostats on the exhaust fans and to block up the intake vents.
The result is, although it is cool tonight I want the fans on so the poly is now sucked tight onto the greenhouse frame, none of that destructive flapping anymore.
The fans are overspeeding just like you hear when your vacuum cleaner nozzel is plugged but they are thermally protected so I hope they will be OK. (yeah I know they will because I have accidentally done this before, turned on the fans and forgot to open vents, and it makes your ears pop if you are inside, and you can't open the door!!! :) )
Anyway, just an idea for anyone with a poly covered house in a weather emergency.