300 tdi disco ..

If i recall correctly it did used to lock the doors until i changed the discos battery, ever since then the fob has only unlocked the doors.
No biggie as the key locked the doors fine, untill yesterday and the key just spins doing nothing.

I can still lock all the doors by going through the rear door but any idea why the fob would unlock it but not lock it?

Have a look the master central locking actuator in the drives door, they have been known to require a simple readjustment.
So if an adjustment is required slightly released the four screws on the actuators mounting plate and move one way or the other 1mm at the time untill u have full locking unlocking remotely.

Although u may find another cause after u have removed the door card, a faulty actuator for example but it could be crystal ball time.