You only get cash back on inflated price
I have looked at booking space at NCP at Glasgow Airport. Tried direct, got a price and an offer of 5% discount. When I input discount code price offered by their web site price did not change. I tried Quidco and the advertised price was £20 dearer than going direct. That is why they can offer 15% cash back - they are still making an extra £5 on the standard rates. The product they sell for airport parking is excellent. but their business practices leave something to be desired.