Dismantling

You'd be right in thinking that the housing splits around the edge into two halves. There is just one small screw hidden behind a circular tamper evident sticker. You'll need to get your thumbnails in the cracks and prise it apart. It can feel very stiff sometimes, almost as if it's glued, but then it will spectacularly explode into a million pieces, well, nine in fact. The two halves, one with the PCB still attached, and then the seven buttons. These can all be put back in of course on assembly but not if they have gone up the vacuum cleaner first. I thoroughly recommend you open this unit up over a table or bench.

To remove the PCB, you need to unsolder the battery connectors, it's physically bonded to the back case half by this method.

The reason it is hard to come apart is that there are a couple of long pillars and deep pockets which locate the front and rear case halves toward the top of the unit. You need to prise away on the lump where the signal lead emerges.