I haven't met this location before.
Two council decisions are required before footway parking can be tolerated.
A resolution of the council itself, which I don't think exists.
A traffic order to give effect to the resolution.
The traffic order is map based and footway parking is shown by a bay which is drawn partly on the footway.

Having had a read of chapter 13 of the Traffic Signs Manual Chapter 3 I think there should be signs and this is probably the best representation to make that the council have not put up signs which show the extent to which footway parking is allowed and given the lack of clarity the PCN should be cancelled.
As always, to win you have to forget about the discount and be prepared to go to the tribunal if the council reject your representations at the PCN and Notice to Owner stages, as they probably will.