Robbers got away with £65,000 worth of rings and Rolex watches from a Harpenden jewellers in a scam based on a bogus wedding.
Jamie Kalaiarasu posed as a customer called "Jay" to ensure the jewellery was in stock before he and co-defendants Ashley Latimer-Bassill and Daniel Stanley kidnapped and robbed the manager.
On Friday Kalaiarasu, 22, of Archers Road, Stevenage; Latimer-Bassill of Loxley Court, Crane Mead, Ware and Stanley, 24, of Travellers Lane, Hatfield, all pleaded guilty to conspiracy to rob Michael Barres on March 20 this year.
They were jailed by Judge Stephen Gullick at St Albans Crown Court for a total of 17 years.
Prosecutor David Chrimes said Loveweds Jewellers in Harpenden High Street has a single show room where customers can view replica items of jewellery before they are ordered from Hatton Garden in London and send to the shop, where they are kept in the safe.
"Jay visited the shop a number of times, building up a rapport with Mr Barres. They spoke 7 or 8 times in the 10 days before the robbery. Jay told Mr Barres he was having a very expensive wedding and wanted to order some expensive jewellery - rings for his wife and the bridesmaids and Rolex watches for the groom and the men involved. It came to £65,000," said Mr Chrimes.
On March 20 Jay made an arrangement to collect the jewellery and went to the shop at around 4.30pm. He established that the jewellery was in the safe before leaving without taking it or paying for it.
Mr Barres locked up half an hour later and it was then that he was kidnapped by Latimer-Bassill and Stanley. One said: "This is what is going to happen." He was escorted to a car and had a cotton hood, like a pillow case, put over his head and his hands were tied with duct tape. Although he could see not see a gun, he was told by the robbers that they had one.
Kalaiarasu came to the car and was recognised through the hood by Mr Barres as Jay. He was made to hand over the keys to the shop and give the details of the alarm code and the safe. Kalaiarasu went in and returned later with the £65,000 worth of stock.
Mr Barres was driven around in the BMW before being freed. He was told to sit in his own car for 10 minutes before doing anything. He was warned there was a sniper in the area and he would be shot. He went into a nearby pub and called the police.
On an earlier visit another member of staff was suspicious of Jay and had taken down his number plate. This led to the arrest of Kalaiarasu. The two others were arrested shortly afterwards.
Around a third of the jewellery was recovered. None was at Kalaiarasu's address, some was found at the other two defendants' homes.
The judge was told Kalaiarasu had four previous convictions. Latimer-Bassill had served a 40 month sentence for wounding. Stanley had no previous convictions, but one caution for aggravated vehicle taking.
For Kalaiarasu, Patrick Dorian said his client was genuinely remorseful and had pleaded guilty at the first opportunity. He had the support of his family and girlfriend. At the time of the offence he was working as a chef in a care home and had met the other two defendants.
Alison Morgan for Latimer-Basill and Stanley asked for credit for their guilty pleas at the first opportunity saying the robbery was "not sophisticated."
Judge Stephen Gullick jailed Kalaiarasu and Latimer Bassill for six years and Stanley for five years. He told them: "No doubt Mr Barres was terrified by these events." He said the robbery had been pre-planned and the victim had been alone in the shop and was vulnerable.