The sign on the opposite side of the street is irrelevant, as it only applies to that side, and not where you parked. This is the basic rule for signage; only signs on the side where parked apply.
Looking at the sign for the bay where you parked, it seems fairly close to your car and is also (a key feature) attached to the sign for the bay. So, reading your narrative, it seems you erroneously took the sign on the opposite side of the street as applicable to your side, and (1) didn't go an look at the sign for the bay where you parked, and (2), never saw the suspension sign on the bay sign where you parked, correct ? If so, you are going to struggle, because it does appear as if the council have signed the suspensions correctly, and you have misunderstood the rules on signing parking bays and suspensions of parking.
So, looking at your question : -
I note the LBHF Cancellation Guidelines spreadsheet suggests the following criteria for a code 21:
Suspension sign error
Suspension sign missing or not visible from where the vehicle is parked
Would either be applicable in this instance? Or any other ideas on how best to challenge this?
In my opinion, neither applies, and the suspensions seem to be correctly signed. However, that is my opinion, so wait a bit and see what the others say, but don't miss any deadlines for paying or submitting reps on the PCN.
The PCN looks OK and correctly made out.
