An enlightening article @Marchbanks - thanks
Based on the article, that door has tension bracing on each half. The 'preferred' solution seems to be one tension brace plus one compression brace - with the compression brace at the top. I can see there is some logic to having one of each - but the next question of course would be why one would choose to fit a compression one on the top as opposed to a tension one, or vice-versa ?
Yes and I believe it is much debated in joinery circles. I am only an amateur (as you can see from the one door out of the 8 that I got the 'wrong' way round). The idea as I broadly understand it is that the diagonals running from the bottom hinged corners to the top unhinged corners will stop the outer (non hinged) side of the door from sagging/dropping. However, Marchbanks's article shows that this is not a universally accepted 'thing'. I did, however, change that door to match the others and none have sagged as yet.