If it gets to a judicial review put your money on Asda. Judicial Reviews are about who can pay for the best legal team.
Who would you say that would be Stockport Council or Asda?
A JR can only be pursued by a party if they can demonstrate that the Council acted improperly whilst making their planning decision or did not follow set procedure. It is absolutely not about the rights and wrongs of the planning case. In any event, all a JR can do is quosh the original decision and effectively demand that the process be repeated correctly. A JR cannot reverse the decision in itself. All the Council would then need to do is simply repeat the process correctly and make the same decision again.
Typically a JR would also be lodged post the original Council decision, not after an appeal - as you only have 6 months post the procedural error being known to lodge a JR. You can seek to JR an appeal, but its a fairly pointless process for the reasons set out above - an Inspector is not going to change his original decisions just because the JR proved that he got his paperwork slightly wrong!
IMHO JR would be a fairly pointless approach for a supermarket to follow in the case of the H.L. site anyway, as the Council has already nailed its colours to the mast re: its decision. The only way that a supermarket is going to win is via strength of argument at the planning appeal stage.
Sorry for a very dull and boring post....