I would get in touch with your local councils parking office and ask them what you should do first.
Here in Brighton we have it pretty good in that we get a free "window cleaner on duty" badge that we apply for annually on submission of a current liability insurance document.
I believe this is due to the fact that there is still a local by-law that allows window cleaners and sweeps to park outside the property they are working at.
However the terms now state that we cannot park in resident bays or pay and displays, we are actually advised to park on single and double yellows ! with the town now virtually absent of any free parking this now basically means to legally park I have to park on double yellows on the corner of junctions ! obviously I dont always as I have respect for other road users and pedestrians safety. Most wardens have an ounce of common sense and some discretion about them, more than can be said for brighton and hove city council.
I know none of this helps you, I just wanted a rant because I recently got a ticket for being in a resident bay whilst at the rear of a property. I am appealing it on the grounds of h&s issues and the councils irresponsible advice to park on double yellows on corners of busy junctions. I shall let you know if I'm successful
Simon