Thanks to Joe and Hash for replies.
I have now discovered by accident (and this is a change from pre new battery), that I have to press 'lock' on the key fob twice to lock both the front doors then the rear doors. Likewise, I have to press 'lock' on the key fob twice to unlock both the front and rear doors. The pattern seems to be: press once to lock the front doors, then press again and to lock the rear doors. Very strange!
As I say above this is a change from before I had the new battery put in. In fact. its a sudden change since the car was new. Before I only needed to press either lock and unlock once on the key fob to lock and unlock all the doors.
Hash - don't know if this helps you or not. Before I replaced the battery. I did notice that I sometimes had to press the key fob several times to get the doors to open and also that the auto seat position set in the key fob didn't work as well as it used to.
At least I now know how to lock and unlock the doors!
Thanks again to everyone for your help and ideas.
Roy