I had not parked when I received the PCN, I remained in the vehicle with the engine running. Am I still liable to pay?

Waiting in the vehicle and parking are the same thing.

See Key Case: Review decision of Schwarz v Camden (2001) PATAS 2110000692, which considers the definition of ‘parking’ with reference to the Road Traffic Regulations Act 1984 and Strong v Dawtry (1961)1 WLR 841.