“regulation 26 should be understood as allowing supersession to be carried out where a relevant change of circumstances cannot be identified. Whether there is an identifiable change of circumstances should thus be considered first, however briefly. Regulation 26 should be considered next if and only if no change of circumstances (or other alternative ground of supersession) has been identified. In effect, regulation 26 is a provision of last resort for cases where no other ground of supersession is made out.”