I don't know if your car is different in the UK than those in the US but I would start at the wiring harness to you rear hatch. On many cars with this, instead of a sedan with a trunk, the wiring wears from flexing over all the times you open and close the hatchback to get in and out of the car. If some wires are rubbed through enough to touch electrically, it could cause lights not to work, the electric locking circuit , other things that tie into wiring in other areas of the car. I know on my Son's Escort wagon that his latch for the hatchback went bad, causing the fuse for the electric locks to blow, once I replaced the locking mechanism and fuse, everything worked again. Did you have the battery load tested? Does the car start normally, did it with the old battery too? If you are having issues while driving with the electronics in the car, your alternator might be the cause too, not sure of the terminology in the UK for some of these things. I know if the alternator is not putting out enough power that electronics, like the ECU for the car, the radio, etc, will act really odd. Sorry if this is a lot, more came to mind as I was typing.