Key went on the blink, replaced both the batteries, red light comes on key and UV shows onn mobile phone camera but key will not operate centrallocking or immobiliser. Where have I gone wrong? Car is automatic transmission. Spare key still works but I feel vulnerable with just one operating unit,

Keith Young, March 2012

In the car, press the central locking button (the one on the dash/by the handbrake depending on model, not the one on the fob) and hold it in until the doors lock and unlock, usually about 5 seconds.
Release the button.
Quickly (within 10 seconds) point the keyfob at the receiver (above the rear view mirror) and push the button on the fob. Doors should lock and unlock, job done.
You might have to repeat a few times.

frank, March 2012

Probably need to resync it, have a look here

http://www.keyfobrepair.co.uk/Key_fob_Resynchronizing_instructions..htm

SD, March 2012