Your mother* isn't at adjudication, she's yet to make risk-free informal reps which guarantee that the discount would be re-offered.
What I see as being the situation... Don't use it verbatim!
She parked; she saw the traffic sign ahead(as can be seen in the CEO's photos); she went to it and followed its directions, see payment receipt.She did not return to the car(parking rights being virtual).
She missed the end of bay marking because another car was parked on it as can be seen in the CEO's photo NB. not the one taken from the carriageway as my mother who is ** years old is not in the habit of wandering into the middle of the road to look at markings, but instead finds it safer to stay on the footway from which vantage point the markings were invisible because the grey car was wholly above them(if it had parked any closer to the kerb it would have been on the footway). Had the council followed best practice, including that set out in Chapter 3 of the Traffic Signs Manual, and placed a post with 2 signs on a single column indicating the point at which the restrictions changed then this issue would not have arisen.
I got her to try and take a good picture of the car obscuring the line but her camera skills aren't the greatest! Then I suggest you do it! You need the lines from the vantage point of the footway, in particular the 12-18 inches measured from the kerb.
