Bruce,
Thanks for looking and for some good points.

The bridge voltage (R1/R2) is either 0V or 4V. I used analog as I may need to be flexible on the voltage values later on.

The sleep operation does work fine, but I have disabled global interrupt at the start now.

The problem is that GP4 sometimes goes high of its own accord on switching on. The length of time it stays on varies, after that operation is as I intended. Is this a boot up procedure I have not reckoned with?

Also, have I assembled the bytes of the 10bit A/D correctly?

Cheers