A Sussex charter boat skipper was starting a five-year jail sentence today for his part in the failed Millennium Dome gems heist.

He was one of five men convicted over an audacious plan to snatch £200 million worth of diamonds from the Dome in Greenwich, London.

If it had worked, it would have been the world's biggest diamond robbery.

Four men found guilty of plotting to carry out the robbery were yesterday jailed for between 15 and 18 years.

Kevin Meredith, 35, of Aucklands Drive, Brighton, was cleared of conspiracy to rob but convicted of conspiracy to steal and given five years.

Raymond Betson, 40, of Chatham, Kent and William Cockram, 49, from Catford, south-east London were each jailed for 18 years.

Robert Adams, 57, of no fixed address, and Aldo Ciarrocchi, 32, of Bermondsey, south-east London, received 15 years each.

The four had admitted a lesser charge of conspiracy to steal, which carries a maximum sentence of seven years in jail.

The gang tried to snatch the stones by smashing their way into the Dome on an earthmover and escape across the River Thames by a speedboat driven by Meredith.

They ram-raided their way into the Thames-side tourist attraction on a JCB but were caught red-handed by armed police inches from seizing jewels from the De Beers diamond exhibition.

Jailing the men at the Old Bailey, Judge Michael Coombe told them: "You played for very high stakes and you must have known perfectly well what the penalty would be if your enterprise did not succeed."

The Millennium Star and the 11 Millennium Blue Diamonds, ranked among the finest diamonds in the world, had attracted thousands of visitors to the exhibition.

But unknown to the robbers, the real jewels had been swapped with fakes after a police tip-off. The raid was foiled by the Flying Squad, backed up by armed police.

The jury of seven women and five men reached majority verdicts after deliberating for nearly seven court days.

The gang had caused terror as they sped towards the vault in the JCB on November 7, 2000.

A boat, skippered by Meredith, was waiting to take them across the Thames to a getaway van. But they ran straight into a police ambush.

As the judge passed sentence, only Meredith showed any emotion.

Referring to his defence, Judge Coombe said: "In my years at the Bar and bench I have never heard a defence of duress with less merit or substance."

He told Meredith the jury had taken the view that he was not one of the original conspirators and there was "a real possibility that you did not appreciate it was a robbery rather than a theft. You were not in at the planning stage, but you knew you were taking part in some serious crime."

Last night, Meredith's wife Vanessa said of her husband's sentence: "It is a bit steep."

She said he would appeal.