Disputing penalty charge notice?

I live in a residents permit road. Yesterday I had a worktop delivered to my house-the workmen parked up, crossed the road and knocked on my door, I promptly went upstairs to fetch visitors permits and by the time I got back downsatirs(literally less than 1 min) a traffic warden was putting a ticket on their van! Surely people have to be allowed time to get the visiotrs permit? Do you think I have a good case?