Captain Custard - Unless you have the METAR read out to you prior to starting the approach, I don't understand how you would use a METAR QNH anyway. I use the RAAFs Flight Info Handbook vice AIP, and I've never used a METAR for anything other than checking the actual conditions before stepping for a flight. I guess if a METAR was given to you just after it was produced, you could use the QNH for 15 minutes (the validity of an approved source QNH) as the METAR was presumably produced by an approved MET observer.
DrP.