Definitely try the leisure centre, but you may be wise for your tone to be dictated by the nature of the situation (about which we currently know very little).
The alleged breach is that the driver failed to pay for the duration of the stay? Is that the case? If so, you might be wise to be somewhat gentler with your tone, as you would essentially be appealing to their good nature. If it's the case that the charge shouldn't have been issued at all, you may wish to be more firm, but bear in mind your email will be read by a human being, who may be more amenable to helping you if you are firm but polite.