Defender TD5, Battery Replaced, Alarm Goes Off

Hi, I have a 2004 Defender 90 TD5. Just replaced the battery, the old one was completely flat. I followed the manual to turn ignition to position 2, back to 0, remove key and disconnect battery within 15 seconds, not sure if this worked as battery was completely flat. As soon as new battery is connected alarm goes off and I can't stop it with the fob or keys, only by disconnecting the battery. Is there a solution? Thanks.

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