Rob Bone, of West Midlands Police, uses his expertise about knots to establish whether a person was murdered, held captive or deliberately tied themselves up.

His knowledge has been called upon to solve cases around the world - from a series of sex attacks where women had been tied up in Australia to the death of Russian tycoon Boris Berezovsky.

Mr Bone's interest in untangling such cases was sparked by a colleague's vast knowledge of knots when he started working in forensics in 1987.

With more than 4,000 different kinds of knots, he said the skill is not in identifying them all but in working out how they had been tied and what materials had been used.

"Knot examination can help show if someone has bound themselves - maybe to stage a burglary - or if there was genuine third-party involvement," Mr Bone, who is based in Birmingham, explained.

"The signs I look for is how secure the knot is, what material has been used, and are there signs that whoever tied it has knowledge of knots, for example a rock climber is likely to use different knots to a fisherman.

"You look at whether a person could get themselves free: if someone can't escape it would indicate a lot of time and effort has been put into creating the knot.

"But the key is to be objective...it is about putting what you see into the context of the case."