HERE’S a ghost story for Christmas, and it all started 170 years ago this Friday. It was in the early hours of December 23, 1841 that Police Constable Thomas Lamb disappeared from his beat in Huntingdon.

He'd been on patrol as usual until about one o'clock in the morning when he was seen talking to a passer-by. A few days later his hat was found floating in the river at Hartford - police uniform in those days included a top hat rather than a helmet.

The river was dragged but nothing was found. Pc Lamb's disappearance left Huntingdon rather short of policemen. The Watch Committee had recently sacked another constable for "partaking of Beer and Meat at the Sun Public House".

The Huntingdon police regulations prohibited officers from entering a pub except in the course of their duty. They could also be dismissed for "intermarrying with the family of a reputed bad character".

The mystery of Pc Lamb's disappearance was solved two months later when his body was found under Huntingdon Bridge. His heavy greatcoat must have dragged him straight to the bottom of the river.

How did he get into the water in the first place? We'll never know. The Cambridgeshire Constabulary Roll of Honour, where his is the first name listed, says that "it is believed he was assaulted by several persons and thrown off a bridge into the river".

He had been due to give evidence in a case of assault to be tried the following week, which might have given someone a motive for getting him out of the way. But the case didn't seem serious enough to make a suspect commit murder.

At the inquest held after the body was found in February, a witness spoke of seeing Pc Lamb standing on Huntingdon Bridge on the night he disappeared, leaning his head on his hand and looking down into the water.

His body showed no marks of violence - though they might have been hard to detect after two months in the river - and his truncheon was still in his greatcoat pocket when his body was found. So he might have committed suicide, or he could have been taken by surprise by several people and bundled over the parapet.

The only way to find out is to ask him yourself. His ghost is said to patrol Huntingdon Bridge to this day.