In the end I found the relevant Lambeth email address by messaging the Lambeth Twitter account. They advise: "Our parking team can be reached by email using environment@lambeth.gov.uk as this goes directly to the team for them to investigate and advise."
So ... have emailed this department now:
Lambeth Parking Services
Dear sir/madam
Penalty Charge Notice: XXXXXXX
Vehicle registration number: XXXXXXXX
Date of offence: 25/10/2024
Date of Charge Certificate: 23/12/2024
I received the Charge Certificate by post on 3/1/2025, and to avoid a threatened county court case I quickly paid the £195 in full.
However I believe my payment was made in error. I want to appeal the matter and misunderstood where I was in the process because I did not receive the original Penalty Charge Notice in the post.
In fact I did not receive any PCNs or reminders issued earlier, and as such I had no opportunities to make an early payment.
Please reopen the case and refund the payment pending the outcome of a Statutory Declaration.
As I said, I believe my payment was made in error and my bank HSBC confirms it is still pending. Please cancel the payment from your end or otherwise refund as I intend to follow the prescribed procedure to deal with this matter.
Thankyou for your understanding.
Yours sincerely
XXXXXXX