By Angus Crawford Home Affairs Reporter

It was a dignified remembrance

They were the only things which broke the minute's silence at this morning's service for a London police officer shot dead 20 years ago.

In the centre of St James Square opposite what was the Libyan embassy stands a flowering cherry tree.

Beneath it a white memorial stone. "Here fell WPC Yvonne Fletcher". A polished phrase for what was a brutal killing.

In front of it, her mother Queenie and father Tim. Beside them Baroness Thatcher, frail but dignified and dressed in black.

Do your duty in all things, You cannot do more. You should never do less Sir John Stevens Tribute to murdered officer

The new members of the blue line, still protecting London. Opposite them across the square their predecessors.

Hair now greying, waistlines not quite regulation issue, but standing quietly to attention, immediately recognisable for what they once were.

A short speech by Sir John Stevens, his voice at times strained and choked, paid tribute to the dead officer.

He borrowed words from the American Civil War's General Robert E Lee.

"Do your duty in all things, you cannot do more. You should never do less."

Then the Lord's prayer, silence and flowers. Noisy, bright and vibrant. As her family said: "As much a day to celebrate her life as mourn her death".

Bouquets of flowers

Suddenly the incessant buzzing of a helicopter - the force chopper hovering over the square, then dipping its nose in tribute and banking away.

Sir John strode to the battery of journalists and declared his intention to trace the killers. A court in London or one in Tripoli, either would do - as long as justice was served.

And then it was over and the square was empty. Bouquets of flowers rustling in the breeze.

But then a squeal of brakes and a black cab pulls up. The young driver climbs from the cab walks straight to the memorial, stops for a moment and crosses himself.