Hi, I had this problem too, the problem was NOT ALWAYS the battery.
In fact the 10th Jan post is correct, it's to do with the gearbox.
In some cars, like mine, the issue is where the gear selector linkage to the gearbox needs greasing. It looks like a universal joint just behind the engine looking underneath the car. These get rusted and stop the gearbox appropriately going into 'P' gear.
It's an easy fix, just raise the front of the car a little (can just go up a curb) and slide under the front of the car. You will see the gearbox, look right above it, around 50cm above you and you'll see a very rusty universal joint. Give this universal joint and the linkage towards the bulkhead of the car a very liberal squirt with grease (I used basic Halfords grease).
Once greased, move the shifter around from B to P, up and down a few times, put the car in P, push the car a little (if it doesn't register that it's in P) and it will register it's in P. Keep moving through the gears, apply for grease if it's not smooth. And you'll have fixed your problem!
I hope this helps some people out there, I can't find anywhere the above fix, I got lucky and discovered it myself after needlessly replacing my battery, twice!
Good luck
