Hm, difficult to say what to recommend.
Whilst the approach passed a sign that allowed off-carriageway parking after it was passed, the car was parked past another sign that was the limit of the allowed parking area, and was, in fact parked only a few feet away from this sign. So whilst she saw the first sign as she drove past, one must ask how she avoided seeing the other sign as she drove past that as well. An adjudicator seeing where the sign is and where the car was parked is, in my opinion, not likely to favour the appeal !
So, not a strong case on the basis of where parked in relation to the signs. However, past cases like this have often found that the council resolution allowing off-carriageway parking only lists the street names and not the actual lengths of street where the parking is allowed. Appeals at London Tribunals have been won on this, but I haven't seen one for some time now, maybe others have, or have access to the street plan. I think 'Stamfordman' may be able to help you with this. Certainly on the basis of the actual contravention, there really is no robust appeal case.