Underneath your key guard there is a ring that activates/de-activates the immobiliser.
Sometimes this slips down from sitting snug up at the top. Undo the nuts that hold it in place and check the connection
and that is still tight up the top in place. Usually a little electrical tape can help to hold it in place.
If that is not what it is then you might need to start trouble shooting. To me it sounds like a common-ish little niggle.
Mine did the same from brand new, the key Aerial wasn't fitted properly as like Matty says here.
Easy fix & has been perfect ever since.
Thanks Matty
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