It is the owners' responsibility to look after their own properties

Williams Evans, Isle of Angelsey Council

The design fault came to light when this tenant became ill last week, after the council installed cavity wall insulation. But people who have bought their homes from the council are angry that they have to pay for the work.

Residents on the estate were told to turn their heating off when the fault was discovered on Friday.

Special cameras have been used to try to see if the defect is a design fault from when the houses were built.

Some residents are undergoing blood tests as a precautionary measure. Homeoeners on the estate said they bought them in good faith from the council and resent having to pay for experts checks to be carried out.

But Isle of Angelsey Council's William Evans said the council had informed all residents of a possible problem with carbon monoxide.

"As far as we're concerned it is the owners' responsibility to look after their own properties."