I will be interested to read how this goes.
I have a PCN following a similar incident after a hospital visit. I am not intending to hijack this thread I have already posted separately - in my case the car reg number is wrong on the PCN so that should be sufficent reason not to pay it.
What I wanted to say is that I think hospital car parks and nearby streets must be really lucritive for Councils and Private companies - with hospital appointments overrunning etc, also if you are having to discuss health issues or visiting an ill person, then parking is the last thing on your mind and you can easily forget what time your ticket expires. I think they are taking advantage of people who are in difficult situations and it makes me so cross. The parking wardens around our hospital are like vultures, they hang around just waiting for the parking tickets to expire, must be a pot of gold for them.
Sorry I don't have any specific advice, I just wanted to say good luck!