Geoffrey Crow, 48, from Bedfordshire sustained fatal crush injuries in the incident at the former RAF Chenies site on 13 February 2012.

The dumper fell into a deep and unguarded excavation, overturned and landed directly on top of him. He was killed instantly by the five-tonne machine.

Harrow-based Lois Gastoneaux Ltd and Michael Brett, a self-employed contractor working on the site at the time have been sentenced following an HSE investigation.

St Albans Crown Court heard that Crow was working at ground level whilst others were working to excavate a deep basement for a swimming pool at a new build property belonging to the sole director of Lois Gastoneaux Ltd – Kevin Andrews.

The dumper went into the large excavation, which was up to 6.5 metres deep, shortly after he had freed the vehicle when it became stuck near the unguarded edge.

The HSE found that despite operations being underway for three weeks at the site, there were no measures in place to prevent people or vehicles falling into the excavation, or to prevent any collapse of the excavation faces onto those working below ground.

None of the workers on site were used to operating plant machinery, neither did they have relevant construction experience despite being tasked with digging such a large excavation.

The seat belt on the machine Crow was driving was not operational at the time of the incident, and his colleagues also stated they would not usually wear seatbelts when operating the machines.

The court was told the range of issues were all contributory factors in the death, and that standards at the site fell well below those expected.

Lois Gastoneaux Ltd, from Harrow, Middlesex, was fined £150,000 and ordered to pay £28,033 in costs after pleading guilty to safety breaches.

Michael Brett, of Little Chalfont, Buckinghamshire, was fined £2,000 and ordered to pay £1,500 in costs after pleading guilty to safety breaches.

Speaking after sentencing HSE Inspector Stephen Manley, said: “Working with construction plant can be extremely dangerous, which is why appropriate safety measures must be in place at all times to protect workers and others onsite.

“In this instance, Mr Crow died as a direct consequence of the lack of controls of the risks involved in the excavation operations. There was no protection whatsoever to ensure workers, whether driving machinery or otherwise, did not fall into the deep excavation.

“A number of people were at work with Mr Crow and they were all at risk of serious harm through the absence of physical controls, as well as poor maintenance of equipment and a lack of training and information provided to workers.

“There are clear industry standards setting out how to identify and manage risks, and guidance is widely available. So there is no excuse to let operations continue without having the proper health and safety measures in place.”