Sir Robert Mark, the thinking man's police commissioner, retired this week convinced that Scotland Yard is a cleaner place.

He carefully does not say that his Yard is cleaned up because Mark prides himself on blunt, honest candor.

"Corruption was routine in the Criminal Investigation Division, (CID)" the 3,000 plainclothes detectives, Mark said the other day as he was cleaning out his eighth floor office. "I can't claim to have eradicated it altogether. But this is the first time in 100 years that the Metropolitan Police Force enjoys as high a standard of probity as any other English organization."

Even as Mark spoke, six CID men, from constable to Detective Chief Superintendant, were on trial for shaking down Soho's pornography dealers for more than $10,000 over eight busy years. Another five, including three Detective Inspectors, went behind bards for from four to 10 years last December. They too had been fleecing the dirty booksellers.

Indeed, the Yard's Obscene Publications Squad had been protecting the Soho merchants from arrest for 20 years, tipping them off to raids and selling at cut-rate prices the materials seized from uncooperative stores. But Mark, who came in as commissioner five years ago, has broken up the game.

THe most important of his reforms ended the astonishing practice under which detectives alone investigated crooked or "bent" detectives. Sir Hubbert set up a crack squad, A. 10, of both uniformed and plainclothesmen to pursue all complaints against police. This technique is almost sure to survive his departure.

In addition, he broke down the exclusive isolation of detectives, once a force within the force. Now, most report to uniformed divisional commanders. Moreover, every detentive promoted to sergeant or inspector must serve a turn with the uniformed men.

One reason a vigorous Sir Robert is leaving at age 60 is because the government has said, in effect, his efforts are not enough. Former Home Secretary Roy Jenkins put through Parliament bill providing for a civilian review board, an outside agency to examine charges that the police dismiss.

Sir Robert scornfully says the civilian review board threatens to "reduce the level of police discipline to the level of criminal justice," for which he has a low regard. He insists the board will encourage more police wrong-doers to press their cases.

To an outsider, his argument seems thin. It does demonstrate that Sir Robert, however, is a loyal cop, fiercely jealous of his force's autonomy.

Characteristically, he cites as another reason for going a conviction that he has made all the innovations he could. "I suspect that my inventive capacity is running out," he says, "and that it is time to make way for someone else."

The someone else is David McNee, who has been chief constable of the Strathclyde police force covering Glasgow in Scotland. From all accounts, he is a quiet, insider's man who will not make the waves that Mark stirred.

Most people here will remember Mark for two long running television performances, cooly directing a pair of prolonged and dangerous sieges that ended with a single gunshot.

In one, at Balcon Street, the IRA's premier killer squad of four desperate men held an elderly couple hostage for a week. Mark's patient and judicious squeeze on the quartet ended with their complete surrender and no harm to the hostages. The four are now serving life for six murders, one manslaughter and 16 bombings.

The other great stakeout was at the spaghetti house restaurant where three armed West Indians were caught trying to rob the till. They holed up for five days with the owner and six other captives before they too gave up. No hostage was hurt, but one of the gunmen accidentally shot himself in the stomach.

This affair was particularly tense. London's large black population, angry at what it regards as the Yard's racism, was convinced that Mark would kill the three robbers.In fact, they were brought out without a single shot from the waiting police.

For all Mark's literacy and articulateness - he was hunting for a quote from "Julius Caesar" when he was interviewed the other day - Mark is no civil libertarian. In his new book, "Policing a Perplexed Society," he argues strenously for ending the rule competiting police to warn an arrested person against self-incrimination and for enabling a prosecutor to put a reluctant defendant on the witness stand.

"I don't think the caution has ever been of the slightest use to an innocent person," he says. "It's not a question of the cope doing in a victim. Criminal justice should be a search for truth."

At bottom, Mark, like policemen everywhere, is convinced his men almost invariably arrest the guilty. He brushes aside any suggestion that police given a free hand will wring self-incriminating statements from innocents to clean up the crime sheet and win promotion. He is equally scornful of arguments that an innocent person might have a legitimate fear of testifying. Mark wants juries to know whether defendants have "cooperated" with the police and prosecutors.

Mark's direct attack on corruption and his outspoken manner made him many enemies in the Yard's 21,000 man force. But when he took over, public confidence in the police here was at a low ebb. Now this has changed drastically. Sir Robert Mark may turn out to have been the best friend London police have had since Sir Robert Peel launched a force that became known in his honor as "Bobbies."