Two statutory grounds cover these situations, and they apply to every type of council PCN in England, Wales and London.
You had sold it (or bought it later)
The ground is that you were not the owner at the time: you never were, you had stopped being the owner before the contravention, or you became the owner after it (SI 2022/576, reg 5(4)(b)). If you rely on having sold it, the law asks you to give the buyer's name and address if you know them; if you bought it afterwards, the seller's (reg 5(5)-(6)). London Tribunals says the same for London bus lane and moving traffic PCNs.
Useful evidence: the DVLA's confirmation of the change of keeper, the sale receipt or invoice, the dealer's paperwork, or messages agreeing the sale and collection date.
It was stolen or taken without permission
The ground is that, at the time, the vehicle was in the control of someone who did not have your consent (reg 5(4)(c)). Give the crime reference number and the date you reported it to the police. The same ground exists in Wales and for London bus lanes and moving traffic.
Why the council still wrote to you
Councils get keeper details from the DVLA. If the DVLA record had not been updated by the date of the contravention, the PCN goes to the old keeper. Make representations promptly, within 28 days of the notice, and update the DVLA if it is still wrong.