My guess: For some reason I keep seeing Smith as an absent minded professor type, so I reckon his outfit will be a sort of 50's style academic/Quatermass look, a tweed jacket with leather elbow patches and perhaps a waistcoat/shirt and tie (he'll probably still were trainers or something out of sync with the general tone of the outfit though.) I think he should have a hat and a pipe but both are unlikely given a/ his mad hair. and b/smoking not being a good thing to show on family show these days!
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