I was only just late back to my car after the PCN had been issued, surely there is a grace period?

A PCN is unlikely to be issued if the vehicle has only been left for up to 10 minutes past the permitted / paid for parking period. The enforcement authority is not entitled to issue a ticket until 10 minutes has elapsed from the period of paid for time (i.e. ten minutes after the expiry of your pay and display ticket) or ten minutes after a period of free parking has elapsed.

However, the rules do not provide a blanket ten minute period of grace wherever you have parked as has been widely erroneously reported. The grace period only applies to vehicles that are parked in a designated parking place (paid for or having been allowed time), not for vehicles left on yellow lines or in a parking space for ten minutes without paying.

See Key Case: Chaudry v Royal Borough of Kensington & Chelsea ETA 2160157321).