1997 land rover rangerover 4.0 se auto green lpg

the problem is the immobiliser, my keyfob wont talk to the immobiliser, its a new keyfob, worked last week but now wont, the range rover battery is flat could this be the reason or do i need to find the EKA Code.

You've answered your own question there

the range rover battery is flat

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Without a fully charged battery, you ain't going nowhere. These vehicles do not function at all well with even partially discharged batteries. Get it charged up and try again.
If it's discharging for no apparent reason, do a search on here for alarms / remote receivers / RF receivers etc. There's been hundreds of threads.