I know there was a thread recently about the alarm going off in very strong winds. Mine did it all last night.

Is there any way I can set the immobiliser but not the sensors?

 

If it's the standard alarm then do the following.

click on remote button to lock/alarm car = four flashes
THEN
click on remote button to lock/alar car again = three flashes (motion sensors turned off - this is also what you should do if you wanna lock/alarm the car when the hood is down)

 

The above is correct but you need to click for the second time within 3 seconds of the fourth flash.

 

I thought that too, but apparently when the roof is down the motion detectors are automotically turned off anyway so there is no need. It is needed when the windows are left down with the roof up.

 

...except that my alarm goes off with the roof down despite me pressing the lock button twice