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He now fears that one or more gangs are staking out fairs to identify potential victims from the jewellery trade. He has put up a £10,000 reward for the capture of the gang and the recovery of his goods.

The dealer revealed that he was first targeted on April 12 after leaving a fair at Brentwood in Essex.

It appears that someone had slashed the tyres of his cars in such a way that they did not burst until he was already driving around the M25. When he became stranded on the motorway a car with the gang pulled up behind and attacked, grabbing a case from his car and fleeing. Luckily, they had grabbed the wrong bag and escaped with a pile of dirty washing. The police stayed with the dealer until the AA came to tow his car away in case the gang realised their mistake and returned.

The attack – including the description of the gang – bears strong similarities to an attack the day after, on April 13, at Kempton, not far from the M25 on the other side of London.

In that instance, a gang struck at a petrol station near Kempton Racecourse. Their target was another jewellery dealer who had been standing at Sunbury Antiques Market. He made an unscheduled stop at the garage after discovering his tyres had been slashed. Again, a car pulled up and several men got out and accosted him.

Rugby tackle

“One of them had opened the car door and was running off with my bag,” he told the Antiques Trade Gazette. “I rugby tackled him and he dropped it and they drove off. They even seemed to know just which bag to grab – the one with the small items of jewellery in, not the more bulky stock.”

The Kempton gang was described as all being short, slightly built and of Latin American appearance with Spanish accents. Two were in their early 20s, one in his late 40s and one in his early 50s.

The gang who attacked the day before on the M25 were described as being of Mexican appearance, although the dealer thought that they might have been Eastern European.

The dealer targeted for the second time told how he lost a substantial stock of jewellery at Sandown after being forced to lock his stock in his car when he could not find a porter to help him with his trolley and cases at the end of the fair.

“There was a heavy thunderstorm and I couldn’t find a porter to help, so I had to leave the trolley and cases in the stand while I went to the car and locked the jewel cases in. When I returned with the trolley and cases, I opened the boot and it was empty. Then I saw someone had smashed the rear side window.” The incident was captured on CCTV.

He is now convinced that he was being watched at the fair and is warning fellow dealers to take every precaution on their way to and from fairs.