+1 for having a CO monitor just to make sure.

The house I now live in was owned by boiler/gas engineer in the late 80-through to 2002, he obviously worked on the house himself.

He had a gas fire installed, the surround for the flue is made of MDF, there was duct tape sealing the joint half way up in a bedroom. The gas supply pipe running through the house is(was) unsupported for a span of 13 foot, with multiple solder joints and a 90 degree angle on that span.

This is beyond shoddy/dangerous work by DIY standards, let alone a boiler/gas engineer!

After we moved in we found out from the people living next door (in the other half of the Semi detached house) that he did the boiler work and servicing for the people living there at the time. The young family (2 parents and 2 kids) were all found dead from CO poisoning, wrapped in blankets on the sofa as they'd thought they had the flu.

So rather wary of gas/boilers now and mention it whenever we have service engineers in, just to remind them that fucking up can have real world consequences.

We have multiple CO monitors around the house, don't risk your/your families life for the sake of a few quid for the monitors.