I've done some more research on this, and it appears that the standby button is in the permanently pressed state. This could perhaps explain why the Central is behaving strangely.

Could someone with a working standby button test something for me.

Disconnect the LCD support board (bolted to the back of the LCD) from the main board.

With your multimeter, check for continuity across the various buttons on the button board. You will probably need to remove it from the front panel to do this.

If you are looking at the buttons with the silkscreened writing the right way up, check as follows:
1 2
o o
XX
o o
3 4

The little circles represent the pads attaching the button to the board.

On my Central, pads 1 and 3, and 2 and 4 on all buttons are connected together (not unsurprisingly, when you look at the buttons).

1 and 2, and 3 and 4 are open circuit until you press the button.

With the button board connected to the LCD support board, 1 and 2, and 3 and 4 are open circuit for all buttons except standby. This doesn't seem right to me.

I'd really appreciate it if someone would check their Central to see if their standby button is open circuit across these pins, or closed circuit. My suspicion is that a 'normal' Central will be open circuit.

Cheers,

Andy.