Thanks Richard, Bolt and Ken.
I'm closing in on this problem; I'm about to replace my door locks then try to synchronise the fobs.
I can't find out if this is normal operation, or a fault: When the central locking is engaged with either of the front sill locks, I can't open either of the rear doors from either the inside or the outside, or unlock them with the sill button. There's no reference to this in the manual. Is this normal operation? It doesn't make sense, from a security point of view, that the doors should be deadlocked. I cut the wires to the superlock motors on both rear doors but that doesn't mean that they're not engaging somehow, mechanically.