The posts from equinox, freddy, and hollins have got it right. The threat to existing shops in Marple is grossly overstated. They have been competing with supermarkets for the last 20 years and have survived. They rely on a large band of dedicated supporters, the great majority of whom must already do the bulk of their shopping in supermarkets but prefer to use local shops for specific items for a number of reasons, loyalty, quality, personal service to name a few. Those people will not change to a supermarket even if it is on their doorstep.
I was in Towcester last week, a market town with a smaller population than Marple. They have a Coop supermarket and a Waitrose within yards of the main street, each with its free car park.There is a Tesco superstore, about 3600 sq m (significantly less than half the size of a football pitch for those who prefer a descriptive measurement!) and a Homebase about 400 yards away.
It also has a thriving shopping street possibly due to the fact that local people are not forced to drive elsewhere for their basic shopping