Green’s Dictionary of Slang

(UK Und.) an informer.

1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1977		D. Powis Signs of Crime 177: Car park Informer (nark): ‘Been at the old car park have you?’, or ‘Watch what you say, he’s car park you know’.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.