(Another..) BMW E90 Remote Locking problem

Guys,
I have a problem with an 06 E90 (316). It started intermittently not locking from the remote fob. We switched to the second fob and it started doing the same. Now it won't lock at all from the fob.
These are the normal ones (i.e. not comfort) so the battery cannot be changed (I assume?).
It will unlock from the fob and the boot will open. All doors will also lock from the dash board button. All doors lock when the backup key is inserted in the drivers door and locked.
Other posts have suggested that it is a fuse problem in a lot of cases but it can't really be that here as the locks go up and down just not on command from the fob.
I also have the TIS software and the Access System is not reporting any problem. It seems to say the battery in the fob is good too and doesn't have an issue with the signal strength. I reset the Access System via the software but to no avail
One poster suggested that the system won't lock when the battery is low in the fob. Do the batteries get weary over time - like since 2006? Can that be the issue?

If that is all then I would splash out on a new fob but I don't really want to go to the dealer and have him change out some expensive control unit if that is not required!

Any info appreciated.