How do you get the back off the key fob to change batteries.

I've push and pulled and can't shift it.

2001 Octavia vRS by the way.

It's a flip key?

Open the key up and there is a little tab on the side where the key itself sits when in.

A little screwdriver in there opens it up , then it's a CR2032 I think

Ah, I see, I must have been having a case of idiotitis !

I was trying to slide the two halves apart !

This may help -

http://www.fabia-vrs.com/key-code.htm