Oldlae
The LZ is the same elevation as Aldergrove - perhaps the most reliable way of getting the QFE would have been to set it at Aldergrove and either leave one of the altimeters thus or just note the subscale for setting later - it was not really that far or that long in time from Aldergrove to the Mull so it would have been better than, say, regional forecasts.
If you want to read up on the settings, etc, you need not wade through this thread, just get yourself acquainted with the various documents and transcripts available - basic essentials for anyone with a serious interest in this case.