A plumber who found himself caught short on a job used a customer's water tank to tackle a leak of his own, a court heard.

Roy Williams, 45, of Allways Maintenance Limited, of East Grinstead, was left alone in the loft of a house in Leatherhead, Surrey, to deal with an apparent fault in a heating tank.

Having sent his apprentice back to his van to fetch two valves for the house's hot and cold water tanks, Williams took advantage of the private moment to relieve himself into a white vase.

Jerome Wilcox, prosecuting, told the court Williams poured the contents of the vase into the hot water tank before washing it out in the cold water tank which provided the household's drinking water.

But unknown to Williams, his intimate moment was being filmed by a hidden camera as part of a trading standards operation to catch rogue traders, a jury at Guildford Crown Court heard.

Just a few feet below him, trading standards officers were watching his every move on a screen, stationed in a first-floor bedroom.

Yesterday a jury of seven women and five men watched as a man, identified as Williams, relieved himself into the vase in front of him before striking up a merry whistle and going about his work.

Williams was called out to the house in November 2003 as part of an operation by Surrey Trading Standards Department which had rented the nondescript terrace house for six months and rigged it up with hidden cameras.

As part of an annual covert operation, an officer posed as the householder as tradesmen including plumbers were called out to fix "faults". An expert had set up apparent leak in the hot water tank by simply displacing the lid on to the arm of the ballcock, causing an overflow through a pipe into the back garden.

Mr Wilcox told the jury Williams had sent his apprentice off to fetch two new valves without checking whether the valves in place were faulty. It was only later, when the valves were taken off, Williams was seen to examine the valve from the hot water tank. He told the jury Williams was heard to say "It looks pretty new. There is nothing wrong with it - strange."

The jury heard the "customer" was billed £144 for the work and £29 for the materials.

Williams denies charges of attempted deception, making a false trades description and making a reckless statement.

His company, Allways Maintenance, denies a further charge of making a reckless statement.

Mr Wilcox described the incident with the vase as "a rather extraordinary event".

Colin Topp, a consultant engineer, told the court Williams's actions could have caused corrosion to the heating system.

He said: "For the system to work properly, the water should be inert. If you introduce anything into the system that could promote bacteria, bacteria would grow quite quickly in that heating system."

Shown the section of the film in which Williams was seen washing out the vase in the cold water tank, Mr Topp said: "I certainly wouldn't want to brush my teeth in that system."

The hearing continues.