Been looking at the HP Service Manual and it requires a near full strip-out to get to the CMOS battery. Not sure I have that much confidence in not effin' something up if I go that deep.
Have also noticed that when AC is plugged in there is nothing from the 3 status lights at the front of the unit - Power Status, Hard Disk Activity or Battery Status. Caps, Num lock, trackpad and mute all light up with the power button itself for those few seconds of 'life'. Hmm!
The AC lead was tested for current and seems ok. As not even the charging light comes on could this be power related?