John Stanley had been hired to sort out a water leak between a double chimney stack and roof tiles at a property on Black Swan Lane, Luton in August 2011.

But his team’s repair work blocked the chimney above the gas fire, leading to a potentially-dangerous leak of combustion fumes within the property’s loft space.

Luton Magistrates’ Court heard that employees of Stanley, 47, from Luton, had dismantled the old double chimney to just below roof tile level, made some repairs to roof timbers and felt, then built a single chimney stack back up.

They failed to check the flue for the gas fire in the property’s back living room was still in full working order.

It later emerged that it was this chimney that was blocked by the firm when the stack was rebuilt instead of the chimney of the disused and boarded-up fireplace in the property’s front living room.

The householders only found out about this in November 2012 when they had to call a Gas Safe registered engineer to examine the gas fire, which they were having trouble lighting.

A subsequent HSE investigation found that the gas fire had been used for over a year with all the fumes going into the loft, over their bedroom, instead of through the flue and chimney – leaving them at serious risk over an extended period of time.

A very smoke test would have highlighted the issue straight away after completion of the work.

Stanley, of Hitchin Road, Luton, trading as King Roofing and Durable Plastics, pleaded guilty to a safety breach and was given a four-month prison sentence suspended for two years, and ordered to pay £500 compensation to the couple he put at risk.

Speaking after the hearing, investigating HSE Inspector Robert Meardon said: “John Stanley’s sub-standard work created an on-going breach and a prolonged risk to the safety of a vulnerable and elderly couple.

“It cost them a huge amount of unnecessary anxiety and a considerable amount of money to rectify – on top of their savings they had used to pay Mr Stanley for the work.

“When a chimney is the flue for a gas fire, it is a vital part of the gas installation and should only be done by a competent gas engineer on the Gas Safe Register.

“Mr Stanley advertised himself as a specialist roofing contractor, but he was never on the Gas Safe Register and not qualified to carry out gas work. He should have never undertaken this job knowing that a gas fire was linked to the chimney.

“This incident could have resulted in fatalities as gas fire fumes can contain poisonous carbon monoxide, which can kill. It is vital that building contractors are aware of the risks they create and comply with the laws in place to control the risks.”