Ok you have two button fob, then all you have to do is place it near the starter switch to re-mobilise the engine. Pressing a button will not re-mobilise the system as it's passive. Unless you don't have a disco that is UK spec.
Also fob's do go out of tune and require a refurb which "remotekey" can carry out for you http://www.remotekey.co.uk/
Taken from the owners handbook:
Engine immobilisation is an important aspect of the security system, and includes a feature known as ’passive immobilisation’. This is
designed to safeguard the vehicle from theft should the driver forget to lock the doors and ensures that the engine is immobilised
automatically whenever any of the following conditions occur:
- when the vehicle is locked using the handsetor key.
- thirty seconds after the starter switch has been turned off AND the driver’s door opened.
- five minutes after the starter switch is turned off, or the alarm system is disarmed.
The engine will be re-mobilised automatically whenever the key is inserted into the starter switch, provided the handset is kept on the
key ring in close proximity to the starter switch.