Looks like my central locking motor has bitten the dust in the back door

The problem is that the lock on the back door is different to all the others, so I can't lock the flippin thing with the key - I remember I had a look last time I had the door handle out, and couldn't work out how to change the key barrel - does anyone have any clues on how to do that? - because by far the cheapest way to do it would be a set of 3 locks from eBay and just change the rear one...