There are many reasons why the film is great, but they start with the script. It was written by Barrie Keeffe, better known as a playwright, who had originally been commissioned to do it by Euston Films. Euston was an arm of Thames Television and best remembered now for The Sweeney. Although Keeffe never wrote for The Sweeney, The Long Good Friday bears many of the hallmarks of a Euston production, and is none the worse for that — a thrilling interplay between gangsters and the police, violence, motors, bent coppers, fluent Cockney, the urban backdrop and tremendous wit. Harold, after his Roller is blown up, comments with no obvious irony that Good Friday is an inappropriate time to start crucifying people. He describes his stabbed friend as being taken out "like a raspberry ripple".

Even when Harold is facing overwhelming odds, his jokes keep coming. And Keeffe wrote one of the most memorable scenes in the British cinema, when Harold borrows an abattoir to question other gangsters about what they know of who is persecuting him. The men come in hanging upside down from meathooks, while Harold lectures them on how there has been "an eruption".

The cinematography is stunning, capturing the London waterfront and the East End in a time of transition. The acting is equal to the excellence of Keeffe's script. It is hard to imagine anyone other than Hoskins playing Harold Shand, let alone anyone playing him better. Hoskins exudes a menace and an arrogance that, ironically, combine to make his character a pitiful figure when the seediness of his criminal empire is exposed, and when his inadequacies are laid bare by the IRA and cause the Mafia to abandon any idea of working with him. As with many bullies, he eventually comes up against someone who is an even bigger bully than he is.

Helen Mirren, as his girlfriend, is not a Central Casting gangster's moll. She is from the right side of the tracks — she claims to have played hockey with Princess Anne — but seems to have a genuine affection for Harold that causes her to compromise any standards she might once have had. And Dave King was born to play the part of Parky, the corrupt senior Met officer.

The film is believable because it seems to represent how the underbelly of London really was, and how the people who populated it really were. But it also captures a moment of immense cultural and historical significance. If you have never seen it, I envy you the treat you have in store when you do.