my fear is that certain people will look the other way and not fight for the sorting office! which is a shame! 
...getting drawn in to offering up alternative sites just confuses issues.
Yes but the problem is that any planning inquiry will focus strongly on the alternative site issue. If the supermarket developer can demonstrate that there is no realistic alternative site in Marple for a supermarket, then this will put the Hibbert Lane sit in the box seat. Especially if both sides effectively agree that there is a demand / need for a new supermarket within the town.
MIA do need to understand this. The campaign line of "no new supermarket at Hibbert Lane, but somewhere else is potentially ok", is fine, but will be rigorously tested at planning appeal. There will need to be an answer to the question, "if not at Hibbert Lane then where?". If the answer is not realistic then the case could well be lost.