Just to add for anyone interested, when I was doing a stack of work on the car and it was off the road for quite a while, my battery was nearly flat (loads of opening doors and boot, messing with starter etc) and I managed to lock it and the battery died part way through the central locking, making some dodgy sounds..... it then wouldn't unlock at all using the manual procedure. I was planning to pop the hood and connect jump leads to the battery (already had the front grille off so was going to be reasonably easy to work out how to pop with a screwdriver), but I waited 15 mins then the manual emergency unlock worked and let me pop the hood the proper way