Bugs, tracking devices, hidden cameras – you’d be forgiven for thinking that the secretive world of the private eye was something that existed solely in Hollywood spy films.

Well think again.

It appears business is booming, with more and more firms popping up, offering surveillance and tracing services.

Just last month jilted Maidstone businessman Stephen Williams, from Halstow Close, admitted going to a private investigator to buy a £590 tracking device, which he placed on his ex-girlfriend’s car after their split so he could follow her.

The 47-year-old, who was given a suspended prison sentence for stalking, claimed the PI had fitted the device for him – something which was strenuously denied.

Fees are opaque, understood to start at several hundred pounds a day, and no one knows precisely how many private investigators there are in the UK.

But doing its own investigations into the secretive industry, our sister paper the Kent Messenger has found nearly a dozen firms offering detective services in the County Town.

Among those is National Investigation Agency, which is currently carrying out covert surveillance on a suspected cheating partner in Maidstone.

The boss, who did not want to give his name, expanded into Kent a couple of years ago.

He said: “With the rise of online dating we are now seeing a lot of young professionals wanting us to do checks on potential partners before they meet them.

“You get people saying they are high ranking military officers when it turns out they actually work in Tesco, for instance, or more often than not they are married.

“But around 40% of our cases are matrimonial, wanting to know if their husband or wife is cheating.

“If someone comes to us they already have suspicions, and I’d say nine times out of 10 they are right.”

The firm uses everything from high-tech laptop bugs and GPS trackers to good old-fashioned shoe leather to fulfil their briefs.

It sounds a murky world, but the investigator insists such surveillance is legal – but only if there is consent from one of the owners of a shared device.

And it’s not just cheating partners – the work also ranges from monitoring nannies, tracing old friends or debtors, and corporate clients who want to know if people are lying about being on long-term sick leave.

“Currently our cases range from someone in their 90s trying to find a childhood sweetheart to a couple trying to find out who their daughter’s boyfriend is," said the boss.

“It’s not as glamorous as it sounds. When you’re sat outside someone’s house for a week you feel pretty far from James Bond.”

Lie Detectors UK has a Maidstone base for customers and clients in the area and carries out polygraph tests for a number of reasons.

They are used for pre-employment testing, investigations, employee theft or fraud and even on sex offenders post-conviction.

The examinations record physiological responses to a set of questions and a score sheet is later analysed for truthfulness.

For prospective employees testing can reveal dishonesty, drug use, work history and criminality.

The company has also developed a test to help journalists ensure their sources are trustworthy.

Anybody accused of doing something they haven’t can use the service to prove their innocence.

The London-based group, which has an office in Buckland Road, charges £399 for a polygraph test. Alternatively a member of the team can pay a home visit for £450.

“It’s not as glamorous as it sounds…When you’re sat outside someone’s house for a week you feel pretty far from James Bond” - the National Investigation Agency boss

Lie detectors pick up changes in blood pressure, pulse, breathing and skin conductivity during questioning and claim to have an accuracy rate of 98%.

It is a technique which uses a set of open-ended questions as opposed to the traditional interrogation technique.

Martin O’Halloran was jailed in 1975 for killing Thomas Walker despite denying any involvement in the crime.

Mr Walker, a gay hairdresser, was kicked and punched to death following a night at a pub in Warwickshire.

O’Halloran walked the corridors at Wormwood Scrubs in London at the same time as the Kray Twins.

He was released in 2003 and in 2015, facing ill health, the 68-year-old was determined to clear his name using the test.

He passed on all three questions asked by Mr Hubble.

• Additional reporting by Guy Bell