Gavin Littaur is not the sort of man to take things lying down.

So when the persistent barking of a neighbour's dog started to get on his nerves, he went straight out and bought an ultrasonic gizmo to solve the problem.

Selling itself as 'the humane way to help cure your pet's behavioural problems, his 'Bark Free machine, which was made in China, works by emitting a high-pitched noise that dogs can hear but humans can't.

It comes with a 'bark pickup sensitivity adjustment selector, which can be tuned to the offending dog's bark so it is automatically activated.

What Mr Littaur, of Rundell Crescent, Hendon, didn't know was that the dog his gadget was trying to tame was in fact Rex, an electronic burglar alarm that mimics a dog's barking.

According to his American manufacturer's web site, Rex 'is the ideal watchdog because he does what even the best living and breathing watchdog can't do without some of the annoying problems that go with owning a dog.

"This dog was driving me crazy," said Mr Littaur, himself a former dog owner.

"I was trying to compose a romantic tune on the piano and all I could hear is 'woof. I found this machine advertised in a catalogue. When the dog hears it, I think it slightly disturbs them and it stops barking, which I thought was a jolly good idea.

"But the bark kept going. It was very annoying because it was a monotonous bark. I didn't twig that the reason it was so monotonous was because it was always exactly the same bark each time."

It was only after chatting with the area Neighbourhood Watch co-ordinator that it finally dawned why his expensive gadget wasn't working.

"When my machine didn't work, I spoke to some of the neighbours about it and they said: You nutcase - that's also a machine to deter burglars," said Mr Littaur.

John Reim, the Neighbourhood Watch co-ordinator, of Rundell Crescent, said Rex's proud owner was a resident in nearby Alderton Crescent.

"One of the ways the alarm is set off is if you poke anything through the letterbox. I've put Neighbourhood Watch notes in and set it off," he said. "I don't know if they are very useful. You can time it and anything you can time is a giveaway to a thief."

The Alderton Road resident is understood to have recently moved house - and taken Rex with him.