Most if not all Tesco stores offer 2 hours free parking, you should still be able to get away before your two hours are up. As Tesco control parking using Number plate recognition now so no need to even get a receipt or buy anything it would be very difficult to enforce if you didn't go over the 2 hours stay.
As for the 90 parking spaces for the stadium, that does sound a small amount for the capacity of the stadium, I would have thought you'd need more certainly taking into account spaces for disabled parking right next to the ground.