Strolling from a fashionable city centre restaurant, the man and woman look like any other ordinary couple enjoying an evening together.

Their openly affectionate body language suggests a close and intimate relationship, and the pair appear relaxed and carefree.

However, unknown to them they are being watched and photographed by private detective John Montgomery, who has just caught the man cheating on his wife. Mistrusting partners are increasingly turning to private investigators, like former police officer John, for help in gathering evidence to support their suspicions that a spouse is being unfaithful.

"I have found in my business the richer the client, the less they want to give away their money to a partner who is maybe not being faithful. It is a way of protecting themselves," said John, a confident and strong looking, outdoors-type man.

"Some people call it a peace of mind enquiry. Sometimes it might be peace of mind when we are able to go back to the husband or wife and say there is no evidence of anything, but there are incidents when I have had to give the bad news to people that their suspicions were right. It is not nice handing over the video or photographs of their husband or wife with someone else."

John, who looks very much the action man with his lightly tanned face, casual clothes and rucksack, is bursting with energy and enthusiasm for his job, which he says involves much more than matrimonial work.

The work of an investigator can involve fraud investigations, missing persons enquiries, investigating insurance claims, investigating commercial piracy and carrying out personnel vetting. A number of firms also offer personal security services.

Sitting in a busy coffee shop, John is comfortable talking in general about his Portadown-based private investigating firm Gilbert Law and Co Investigators, but prefers to go somewhere private to discuss the more intricate details of the job.

"I am mostly involved in medical fraud surveillance when a person has a claim with a big insurance company or employer and there are reasons to suspect the claim is fraudulent. Most claims are legitimate, but there are some people who might want to string the injury out in an attempt to get more money.

"My job is to put them under surveillance to obtain evidence that can go before a court to dispute their side of events. The evidence has to be of a good enough standard to be brought before the court.

"I have also done quite a lot of work over the past year and a half with staged accidents - when people stage an accident and don't contact police. They then put in a claim for injuries."

So does spending his days trying to catch people out make John suspicious of everyone?

"I don't always disbelieve people. They are innocent until proven guilty, but if an insurance company, solicitor, husband or wife thinks that something is not right, then they employ me."

The private investigating business is prolific in Northern Ireland. Anyone searching for a private investigator in the Yellow Pages will find almost 40 firms based in Ulster. However, only three agencies in the province are members of the Association of British Investigators, the body that regulates private detectives and ensures they work by a strict and ethical code of practice.

"You have to be careful when selecting a private investigator," warned John, whose own business is licensed by the Office of Fair Trading and Registered under The Data Protection Act.

"Very few are legitimate businesses and do not have the protection of a legitimate business. It is important to be able to check up on the history of a business. With businesses that are clandestine your confidentiality may not be assured."

John's identity has never been discovered by anyone he was trailing. However, he has found himself in a number of dangerous situations.

"One frightening situation was when I was doing an anti-crime op at a major UK music festival. I was watching an ice-cream van that was believed to have been selling drugs outside the event in a car park. I think they caught on who I was and those people carry knives. I saw a load of revellers coming so I got myself into the middle of them to make it difficult for them to spot me.

"If they had have known who I was and had caught me, I have no doubt they would have made life very unpleasant for me. But I guess that is just one of the risks of the job," he said.

Have you a story to tell about a private detective? Contact

dmcaleese@belfasttelegraph.co.uk