Roomster 1.9 tdi 2007. Seems key and immobiliser are not communicating as car won't start, cuts out.  All lights and fuses working. Need to find out how to rectify.  Any ideas please. 

Spare key?

Tried that. Since post tried unlocking manually and car alarm sounded so nothing wrong there.  Cannot see immobiliser red light showing or flickering but it is sunny.

The red light on the door card isn't anything to do with the immobilizer.

When you go to start the engine, does it run for approx 2seconds then engine cuts out and a yellow/orange car with a key light flashes on the dash?

Not a dying battery in the remote by any chance?

Battery in remote plays no part in the immobiliser, only unlocking and locking the car. The immo chip in the key is a non powered transponder.

as above^^^^^^^^^

 

Flashing red light in drivers door just tells you the alarm is active...........the alarm & immobiliser are two different systems (to an extent).........just because the alarm is inactive (off) doesn't mean that the immobiliser is off either! 

Has it been out in heavy rain?

Ensure that you try to start it at least 10 times.

Lock the car up and open the car doors using the key mechanically rather than the remote then see if it will start?

I have a similar problem with the wife's Touran and the above process sorts it every time.

get the car scanned to see what faults you have

it may not have the old style "car with key" immob symbol. The newer canbus models did away with that. Instead it displays "SAFE" where the mileage is displayed.

 

Not sure which a 2007 Roomster would have as i've never been near 1 so just check to see if it has the "car with key" symbol or if it says "SAFE"