Doesn't the 'fine' have to be commensurate with the 'losses' - how can they justify £90 for 10 minutes? Take it to http://www.pepipoo.com/
Reminded me, I heard a supermarket manager talking about how their parking spots are worth 1k a day.
So when you demand a shop justifies a charge with their losses, actually they could give you a base number and explain the costs of the parking company on top.