Britain 's longest and most heated strike was marked by violence again today as hundreds of police and demonstrators wrestled and punched each other at a north London film developing plant.

The clash at the small Grunwick factory here does not involve a great new issue of industrial relations, nor does it raise any important economic questions. There are only 137 strikers. Both the militant left and the hard right, however, have seized on the 11-month-old dispute, turning it into a test of strength.

As a result, the affair has captured an extraordinary amount of press and television converage and has forced itself on the attention of Prime Minister James Callaghan's Cabinet.

About 4,000 police - nearly a quarter of the London force - faced 15,000 outside the plant gates this morning. That is more than 40 times the 429 Grunwick workers involved.

Inevitably, the swollen collided. By this evening, police counted 18 injured in their own ranks, 12 among the demonstrators and 70 persons arrested. None of the injuries were said to be serious, although six policemen were kept overnight in a hospital for observation.

The center of all this fury is a successful, small mail order photo processing business owned by George Ward, 44. A strident union foe, Ward is an accountant of mixed Indian and British parentage and experts think this is a central factor in the dispute.

His workers are largely immigrant women from India, generally among the most vulnerable workers here. Anglo-Indians such as Ward are frequently contemptuous of full-blooked Indians, and his striking workers have complained that he treats them inhumanely.

Last August Ward fired one worker, four more followed him out and soon 137 had joined the strike. No clear picture of wage levels at Grunwick has ever been given, but those outside the plant say that starting pay was about $48 for a 40-hour week.

The striker sought the help of a union and turned to one of the mildest in Britain, the Association of Porfessional, Executive, Clerical and Computer Staff.

Ward refused to recognize or bargain with the union, fired the strikers and said that those who had continued working wanted no part of a union. At first, the affair was a little-noticed local squabble in north London.

It was transformed, however, when postal workers refused to deliver Grunwick's mail, a potentially deadly blow to a company that does more than $3 million a year through the mail. Ward turned to a new organization, the National Association for Freedom.

It won a court order compelling the postmen to end their balcklist of Grunwick. The order has been obeyed only in part, However, and the company still receives less than its normal delivery.

Many union supporters were dismayed by the postal workers' tactics, and this has become one of the side issues launched by the dispute.

Political figures on the far left, notably the International Marxist Group and various shades of Trotskyites, began appearing on the Grunwick picket line.

Their support amounted to little until late this spring, when Arthur Scargill made the Grunwick cause his own. Scargill is the Marxist leader of the powerful coal miners in Yorkshire and is contending for the union's national leadership with Mick McGahey, the Communist chief of the Scottish miners.

Scargill has brough hundreds of miners down to the plant and this had led to frequent fights with police. The police say they must enforce the law against mass picketing and clear the road for Grunwick's nonstrikers to work.

After one policeman was photographed unconscious in the street - he recovered a few days later - scenes of violence at Grunwick became a nightly staple of television here.

All this has forced Callaghan to appount a special court of inquiry under Justice George Scarman, but recommendations will have no binding power.

The entire drama need never have happened. As Joe Rogaly of the Financial Times observed: "It is the natural consequence of a national reluctance to adopt a written constitution, at a time when class conflict is no longer easy to contain by means of unwritten codes that have become anachronistic."

The existence of written rules makes a Grunwick affair almost unthinkable in the United States. The central dispute is the demand for union recognition by the strikers.

In Washington that sort of the question is determined through a ballot by the National Labor Relations Borard. There is a weak equivalent here, the Arbitration and Conciliation Service, but it has no power to enforce its will and no machinery to poll or even determine what is a n appropriate bargaining unit.

From the core dispute flowed such issues as the dismissal of the strikers and raises granted to those who stayed in the plant. All these are potential unfair labor practices in the United States and again would be adjudicated by the NLRB.

As Rogaly said however, Britions prefer "fudge" to explicit rules. Religance on common sense and unwritten rules can work well when passions are not aroused. The right in Britain has turned Grunwick into a Crusade for law and order, for police authority. The left damns the police an instrument of Grunwick's boss and his "blacklegs."