I'm afraid I can't help with the legalities either. First off, I'd take Jack's approach and see if you can't come up with a "creative" solution to siting the extension. Easier said than done, I know.
Also, with something as "high profile" as a supermarket, I'm very surprised that they have added a warehouse and car park without PP. Perhaps it was in their original consent, and they added it later. Either way, I'm guessing that the 8 year period for enforcement has long past.
Trying to take a pragmatic approach, the excessive water discharge/runoff from the supermarket should interest Local Building Control (Building Inspectors - it's part of Planning, but quite separate). It's not usually acceptable to disharge water to another's property.
Even Environmental Health "may" take an interest.
One thing I don't understand is that you mention your garden, then say the Supermarket own the land - your garden?