Thanks Ojuka.
Having worked through all the implications it seems that if everyone, civil and military, standardised on using QNH (as it seems the civil air transport world already has) life would be a bit simpler (and probably safer as a well).
I suspect that the real issue behind the question I've been asked is driven by a resistance to change an ingrained culture to use QFE when approaching/departing. This doesn't seem to be a valid argument as far as I can see. As you've pointed out. it's no great shakes to adapt to thinking in terms of altitude rather than height above a local ground reference.
Cheers,
VP