Free Traffic Legal Advice

The photo of the sign you have posted applies, as it says,  to parking beyond the notice, therefore all sides. These signs are indicating Permit Parking Zones. Please advise us where exactly you parked using GSV, and also your route to get there off the Wilmslow Road. There is no legal requirement for two signs, one on each side, but councils have a duty under LATOR Regulation 18, to place adequate signage indicating a restriction in a Traffic Regulation Order.
Problem is that 'adequate' is subjective. To you it's inadequate, to the council, and maybe an adjudicator, it is adequate.

https://www.legislation.gov.uk/uksi/1996/2489/regulation/18

Wait a bit to see what others say before you submit any representations. Don't just cough-up now,  because if they refuse your reps, (which to maintain the discount must be submitted within the discount period), they'll give you another 14 days to pay the discount. They may also shoot themselves in the foot when replying too !

Please post back of PCN so we can check it for fatal errors.

So here is the sign you passed : -
https://maps.app.goo.gl/ZG67y6Ws4rDdbRxA7
It's clear enough, and you have already sais you took it as applying only to that side, an unfortunate misapprehension. However, best action now is to submit reps on the basis that like all the other street parking notices you are familiar with, you took the sign as only applying to the side on which the sign is mounted. If two signs had been placed, one each side, the contravention would not have occurred.

The FOI may take time to get a response, so best if you submit at least some reps before the discount date expires. That means you will get a further 14 days to pay the discount when they inevitably reject your reps. Once the FOI comes back and if you want to take the matter further, you can submit reps against the Notice to Owner, but the discount will have gone at that stage.

Is your car hired or leased, and is the V5C up-to-date address-wise ?

Yes I’ll be challenging only within the 14 days, regardless of FOI outcome.

Car is leased and lease company charge an Admin fee to deal with any NTO so wish to avoid that

Whilst you can challenge the PCN itself, you cannot submit reps against the NtO unless the lease company pass on your name and address, (and charge a fee, of course !). Your challenge is invariably going to be rejected, so what are you going to do then ?

Surely I still get the option to pay the discounted fine if it gets rejected, right?
I don’t wish for it to got to NtO.

Yes you do, but as we know virtually all informal challenges get rejected by councils, why are you submitting reps if all you're going to do when rejected is cough-up ?

Why would I not exercise my right to appeal, especially if I feel the signage is ambiguous and inconsistent with other signage?

Even if I’m just chancing it.

Rather surprised you appear to be suggesting I just cough up and not appeal at all?

No problem with you submitting reps at all, but councils dine out very generously on rejecting virtually all informal reps. It's only when they have to do some real work for their money that they sometimes concede, and this point is reached at the NtO stage.

Yes.

This is the enforcement procedure for parking contraventions:

https://www.londontribunals.gov.uk/eat/understanding-enforcement-process/parking-penalty-charge-notice-enforcement-process