Just remind these jokers that it actually took an act of parliament to enable the London Councils to clamp peoples' cars. If memory serves me correctly. These characters are relying on ancient common law which goes something along the lines of:- If an animal strays onto your land, you are entitled to hold onto it until the owner pays you a reasonable sum for the grass that it may have eaten. I think that "a reasonable sum" is the key word here, a few private clamping cases have been overturned because the court did not believe the amount to be reasonable. Another thing is that they have to display signs clearly stating how much you will be charged, don't know if they do this at Manchester as I never fly out of there?
One other suggestion; The AA and the RAC have quite militant motorist protection sections that love cases like these, and I think they probably have more resources then Excel Parking of Cemetery Road!!!!!