Smart Parking Fine - ASDA carpark

Hi, I was wondering if anybody could help - I've just received a fine (01/04/15) from Smart Parking for parking in Asda car park (Kings Heath, Birmingham) dated 18/03/15 for parking for over 2 hours (2hrs 40) It was at night and the car park was empty and I didn't know there was any restrictions. It's for £40 if paid in 14 days (so ends tomorrow)

Do I have to pay this?

Please advise what I can do.

Many thanks in advance x

Smart Parking Fine - ASDA carpark