Didn't take the leaflet, but I just got 20% discount on a spend in Co-op (Cheltenham, Montpellier Walk).
They had the details on a leaflet, I'm pretty sure it said it ends on the 21st of October.
Can't find anything online about it either.
Went back to the store for the leaflet. Website here. http://www.midcounties.coop/nusextra20/
Not sure if it is store specific. Hopefully not a goose chase, and helps a person or two.