OK... I done a full analysis on what's going on, and it's got me stumped.
When I turn the air conditioning on via the climate control, +12 volts is sent through the pink wire into the low pressure switch on the receiver drier. Since the system is full, the switch completes the circuit and +12 volts is sent out the other end. The +12 volts can then be seen at terminals 86 & 30 at the A/C compressor relay holder in the relay box located in front of the driver's side strut tower under the bonnet. This scenario is also detailed in this post.
http://forums.justcommodores.com.au/223155-post2.html. It also mentions that the the negative side of the relay coil (pin 85) is supplied by the PCM, which I'm assuming in my VP is supplied by the BCM. Either way, pin 85 is supplying a ground as it should. And finally, pin 87 goes to the A/C compressor to engage the clutch. All the voltages I have mentioned here have been taken with the relay removed.
Now the strange bit. With the relay in place, I heard and felt the relay vibrating, so I re-checked the voltages with the relay's 4 terminals just sitting in position on the relay holder. In this position I could probe the voltages again, and pins 86 & 30 have dropped from battery voltage to 9 volts. At this stage the A/C compressor clutch does not engage. With the relay still in its raised position I applied battery voltage to terminal 86 and the A/C compressor clutch engaged.
With all this information I'm now thinking that the voltages at pins 86 & 30 deteriorated from battery voltage to 9 volts, and it was during this time that I noticed the clutch slipping. Incidentally, the A/C compressor clutch no longer engages at all at this voltage. I can't prove this theory without supplying a variable voltage to the pins, but I believe it to be a fair assumption.
And it gets even weirder. Pin 87, which is the pin that finally supplies the +12 volts to the A/C compressor clutch, has a reading of battery voltage, even though supply pins 86 & 30 are at 9 volts. I got no idea on this one.
Also, if it helps, if I remove the relay and supply a direct +12 volts to pin 87 the A/C compressor clutch engages as it should.
Hope you can wrap your head around all this stuff. If anyone has any idea whatsoever as to what the hell is going on, please let me know, as I'm not going to be able to sleep until I figure this one out.
Thanks.
Chuff.