There are two aspects here,
(1) parking off the carriageway is clearly permitted and signs show this, but the signs do not say "in marked bays". Cases like this have been seen quite a few times, and if the council resolution to allow off-carriageway parking just gives the street name, then the exemption applies to all of it unless there are yellow lines with restricted times.
(2) The council have tolerated parking outside the marked bays for years thus giving legitimate expectation that enforcement will not take place.
So you (and others), need to submit informal reps against the PCNs pointing out parking off-carriageway has been tolerated for many years, (at least 9) without enforcement. Also put them to proof that the marked bays are supported by a Traffic Regulation Order detailing exactly where the bays are.
You might also contact local councillors to complain about council legerdemain, i.e. allowing parking along the whole street then coming unannouced and serving PCNs to virtually all the parked cars without warning.
Having said the above, a look at GSV, latest being 2019, shows many cars parked almost completely blocking the footway which is really asking for it, frankly.
https://maps.app.goo.gl/sttbLxEyAJZFCPBv9