Green’s Dictionary of Slang

1. a gang of thugs.

1955		B. Hill Boss of Britain’s Underworld 7: I had my own mob, probably the toughest team of screwsmen ever [...] We were called the heavy Mob.
1974		G.F. Newman You Flash Bastard 20: The straight businessmen, abetted by the front men like Peter Rosi, had been moving into and dominating areas where firms and heavy mobs once operated almost exclusively by violence.
1989		in G. Tremlett Little Legs 125: Telling people that he was with the ‘heavy mob’ now.
2000		J.J. Connolly Layer Cake 265: That’ll lead to a shitload of complications. Their heavy mob’ll come after us.

2. physically tough police or prison officers used in violent situations.

1954		J. Phelan Tramp at Anchor 167: If the convict were not completely acquiescent the ‘heavy squad’ gave him a tanning.
1956		‘Charles Raven’ Und. Nights 98: Rex shouted: ‘Scarper blokes, it’s the Heavy Mob’.
1962		F. Norman in Police and the Public in Norman’s London (1969) 129: He was a big geezer and was obviously a member of the heavy mob.
1980		J. McClure Spike Island (1981) 384: There’s no doubt we were the heavy mob, we were the snatch crowd.
2000		J.J. Connolly Layer Cake 148: He’d get a serious beating from the mufti, the heavy mob.
2007		N. ‘Razor’ Smith Raiders 27: Bob and Terry were trying to ward off truncheon blows from the hastily assembled heavy mob.

3. (also heavies) officers from the Flying Squad and, formerly, the Special Patrol Group; thus heavy, the car they drive (see cit. 1989).

1956		‘Charles Raven’ Und. Nights 194: In steamed the Law, not the local bogies neither but the Heavy Mob, the genuine Flying Squad.
1957		P. Beveridge Inside the C.I.D. 42: In January, 1940, I was promoted to Chief Inspector and given charge of the Flying Squad [...] or the ‘Heavy Mob’ as it is called by the criminal fraternity.
1959		‘Screwsman’s Lament’ in Encounter n.d. in Norman Norman’s London (1969) 67: We cruised along at a steady ninety, this drag a lovely job, / With Drummer Bill behind the wheel, we’d outstrip the heavy mob.
1961		(con. 1920s) J. Sparks Burglar to the Nobility 27: After me and Gerogie had stolen the famous Wernher Collection of jewellery [...] the Heavy Mob had taken to following Georgie around .
1977		D. Powis Signs of Crime 187: Heavies Police officers from either the Flying Squad or the Special Patrol Group.
1989		J. Morton Lowspeak 72: Heavy [...] a Flying Squad car, usually used in the plural.