Free Traffic Legal Advice

Hi all,
Looking for advice on a PCN I've just received.

Council: Bromley
Contravention: Code 12 – parked in a residents'/shared use bay or pay & display without displaying a valid permit, voucher, or P&D ticket, or without payment
Location: Harwood Avenue
Charge: £160 full, £80 if paid within 14 days (discount period)

The sign at the location shows:
Mon-Sat 8.30am-6.30pm
Sunday 10am-5pm
Pay at machine → Display ticket or Permit holders (A) (Conditions apply)

This was a Sunday. I assumed Sunday parking was free and didn't pay at the machine — on reflection the sign does state Sunday hours (10am-5pm) rather than saying it's free, so I'm not sure this gives me a strong technical argument, but wanted to check if there's anything procedural or otherwise worth looking into (e.g. traffic order, machine/signage compliance, notice wording) before I decide whether to pay the discounted amount or challenge.

I've attach photos of the PCN (front/back) and the sign from street view here -

Would appreciate any thoughts on whether it's worth challenging or just paying the discounted £80 given the timeframe.

Thanks in advance.

In the exact parking spot shown on the Google street view image linked above