Syncing spare key - not working?

[Dohnut]

Only had the car a week, and it will be going to the dealer to get the rear discs replaced as planned so will mention it to them, but if this is something I can fix myself, then I would rather do that.

Basically, the "spare" key which looks like it has never been used will lock the car and start the car but not unlock. I replaced the battery with a brand new one.

I followed the procedure in the manual, well at least tried to. It said to unlock the car with the metal key part after removing the cover over the lock, but again, turning it towards the front of the car does nothing, but turning it towards the back will lock the car.

I just find it odd that it will lock and start the car OK, but won't unlock. Any ideas? Dodgy fob/unlock button, or some other newbie error?

Thanks.