Just followed the advice for my 2005 Zafira. Bought 2 replacements batteries from Ebay for just over a quid.
Once the fob is in half push the battery towards the back of the holder containing the piece of metal spring, be careful not too hard. Battery then pops out. Slipped a new one in and tried it but it didn't work. Put the key in ignition and turned it 2 clicks and pressed the lock button for 5 seconds. Hey presto key was paired with the car again.
Jaytee, March 2011