"If at any stage in any proceedings before a judicial authority … the authority is satisfied, in the light of any considerations which appear to it to be relevant (including any evidence which has been or may thereafter be put before it), that justice cannot be done in the case except by admitting or requiring evidence relating to a person's spent convictions or to circumstances ancillary thereto, that authority may admit … and may determine any issue to which the evidence relates in disregard, so far as necessary, of those provisions."