Anybody know how I can replace the battery in my Octavia key? Thankfully works manually but I was told by Halfords that taking the battery out could immobilise the car (our local dealer closed - 45 minute drive to the nearest).

AFAIK you should be able to swap the battery quickly without a problem.

There is a procedure here for re-syncing the keys - Octavia-vRS.com - Key & Immobilisor Coding

The battery is usually a single CR2032, some of the early Octavias used two CR2016. Stus link above has all the information that you need to replace the battery.

As for Halfords saying that you can render the car immobile by removing the battery, this is totally untrue, the worst that can happen is that you will have to unlock it by turning the key in the lock. The immobiliser is a totally separate system from the remote locking and doesn't need a power source in the key to function.

Halfrauds guff & FUD generator well "up" to its usual standard then!