Claiming that a Pentagon statement did his reputation "a world of good," Henry Catto, assistant secretary of defense, says, nevertheless, that that was no lady arms control specialist he was with in London's Hyde Park last October when 200,000 demonstrated for nuclear disarmanent. "I was just doing my jogs through the thing. I did not see Janet Morgan at all," says Catto of the liberal-leaning English writer and friend of Defense Secretary Caspar W. Weinberger, whom The Sunday Times has labeled "the object of his Weinberger's affection." A Pentagon spokesman has called Weinberger a "devoted family man . . . the No. 1 married man of all time."

Currently traveling with Weinberger in Saudi Arabia, Oman and Jordan, Catto called the breakdown in communications between him and the Pentagon "an error in transmission."

Communications of another sort, a conference on telecommunications, took Weinberger, en route to the Middle East, to England last Friday. As a member of the Ditchley Park Foundation's advisory committee, Weinberger was among 36 conference participants who included Janet Morgan, the Oxford don who edited "The Crossman Diaries" and hopes to do biographies of Weinberger and Agatha Christie someday.

"These are mostly scientific types who talk about what happens when you push buttons," said a Ditchley Park official.

Weinberger arrived at Ditchley, 20 miles outside Oxford, after an overnight flight from Andrews Air Force Base. There had been rumors that he was going to meet secretly with Arab officials at some point during his one-day stay in England. A Ditchley Park official, however, said the defense secretary joined other conferees seated around a long table in the library of the honey-colored stone mansion set among the rolling hills of Oxfordshire.

The official also said it would be "rubbish" to read anything into the simultaneous presence there of Weinberger and Morgan. "He's under protection, remember," said the Britisher. "He couldn't lift a finger with those chaps around."

The Pentagon said last week that Weinberger had known Morgan for several years, since he was an executive with Bechtel Corp.