TD5 Deffender immobilizer issues

Got an odd one going on here. Our TD5 Defender drives great with no issues while running. Stop it & leave it for a couple of mins with or without the key in & unlocked but immoibiklizer kicks in when we go to start it again. Lock then unlock & all is good. Never done this before but started doing it about a month ago. Getting more frequent aswell. Any ideas how to sort this out as not really keen in not being able to start at some inopportune moment or the immobilizer stopping working all together.
Be great if its as simple as fitting a new fob battery..

Have you tried a new battery in the key fob ?

Not tried that yet but gotta be worth a shot. Be great if it was as simple as that.
Thanks Galladog!

Best to try the simplest/cheapest option first.