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Cheers, sarumite.
Had all the rest and I just couldn't remember or find the answer. Ought, as an architect, to have known!
Additionally as part of one of my degrees studied, but not in detail, Byzantine history.
Chuck into the equation that my wife's step-uncle (if there is such a thing!) is an emeritus professor of Byzantine studies, and you will appreciate my frustration and unwillingness to ask for the answer earlier.
Good to have Steff back on line.