would be worth retaining (what a pun) a structural engineer - for the few quid it'll cost you at least you'll know your solution won't crumble away or fall down - I have 2 ret. walls in a house design at present and while I would have thought their design was straight-forward, it has proven to be a little more complicated and so am glad that the st. engineer is there to advise and stand over everything - more money I know but sometimes the structural stuff is worth spending on ...