Hi all new to this but my discovery (w) has immobilised itself, it wouldn't open on the key so i have changed the battery in key and on the vehicle put in the code in drivers door by turning to the bonnet holding for 5sec then opp for number of dig then the other way and so on for all four times then back the other way alarm still goes off any ideas

You're talking about the EKA (Emergency Key Access) code.
A couple of things must be asked here:

1. Is the EKA code you've used the one for your vehicle? It's unusual for the EKA details to be still with a 15 year old Disco.
2. Do you observe the "beeps" from the immobilizer system as you're entering the EKA code or are you just relying on the timing?
3. After entering the correct code indicated by a double "beep" are you allowing the five minute waiting period?
4. After three wrong attempts are you allowing a full ten minutes waiting period before any more attempts?

From what I've read, the EKA system can be very unforgiving of errors.