I cannot Remotly disarm the alarm and the immobiliser?

The car is a mercedes c200 1996.
The remote transmitter has new batteries and is working
as per the checks in the users handbook(i.e depress the button
for approx 1 sec and the red light in the transmitter should glow !). There is no problem with the car battery and the
inginition turns the engine when I tested it.
The red LED in the mirror flashes ok (once every 20 secs)
The only other thing that might have a bearing , is that it
was raining that day, but I have tested all electrics and they
are working ok.