Mark Williams, 41, from Nuneham Courtenay, was using the vehicle to lift a pallet of tiles to a fourth story roof when tragedy struck 20 July 2011.

He tried to flee the telehandler as it began to topple, but he was unable to move away in time and it landed on top of him causing fatal crush injuries.

An HSE investigation found the safety of the vehicle was compromised by limited space and other obstructions in the area where he was required to work.

Reading Crown Court heard during a five week trial earlier this year that Williams, a married father-of-two, was part of a team responsible for tiling a number of roofs.

He was operating the telehandler with the boom fully raised but not extended.

Raising the boom reduced the overall length of the vehicle, however it ultimately caused it to overbalance as it was being turned and manoeuvred.

HSE inspectors established that Williams had no option but to operate the vehicle in this way.

The space between the buildings where he worked was almost the same length of the telehandler with the boom lowered, and meant he would have had no turning circle.

The court was told the vehicle was not suited for use in this area, and that had the space constraints been properly assessed and a better system of work put in place then Williams death could have been avoided.

Costain Limited, of Vanwall Business Park, Maidenhead, was fined a total of £525,000 and ordered to pay a further £90,577 in costs after being found guilty of safety breaches.

After sentencing HSE principal inspector Steve Hull said: “This was a tragic and entirely preventable death. Mark Williams was required to use a telehandler that was wholly unsuited to the confined area he worked in.

“He had no option but to raise the boom so he could turn the vehicle, and in doing so he critically undermined the stability, resulting in the inevitable overturn.

“He should have been provided with alternative, more appropriate equipment and a better system of work. Costain had clear responsibilities to ensure that happened, but they failed to properly assess the risks and ultimately failed Mr Williams.”

Shenda Long, Mark’s mother, added: “I have always felt that I have lived a very privileged and happy life, but all that changed on 20 July 2011 when police officers knocked on our door and informed me that our beloved son had been killed.

“Mark was a loving son, brother, partner, friend and an amazing dad to his two daughters who brought sunshine, happiness, joy, laughter and love into our family.

“Little did we know that fateful day that it would be the last morning we would feel peaceful, happy and complete. How could we know that the simple act of Mark going to work, as he done every day for years, would result in him being killed, and my family’s world ending.

“Mark was totally let down by the people he worked for and trusted.”

Williams’ employer, Attley’s Roofing Limited, was earlier cleared of identical health and safety breaches at the end of the initial trial on 3 April.

The company, of Thorpe Mead, Banbury, was acquitted after being jointly prosecuted by HSE alongside Costain.