In the case of Sadler, it was decided that if the agent (the parking company) has begun a claim against the other contracting party (the driver or the keeper), the principal (the company that hired the parking company) may intervene.
So if you get a ticket and you asked the principal (e.g. the supermarket) and they agreed to cancel it, then this case supports the argument, the parking company must or should have cancelled under the rules of estoppel.
The parking company will counter argue that no-one but they can cancel but a copy of the contract between the parking company and the principal will show this not to be true. The parking company will attempt to deny you access to this contract but you must ask the court to require the parking company to produce a full copy of the contract.
You can download a copy of this case for your defence pack (if applicable) below