Yes, a very expensive solution that I am frustrated at
The Sabre/Vokera engineer said there was nothing wrong with the boiler just the CO levels had been set incorrectly. I don't know what that means. I also don't understand how any thing can be set wrong, when the boiler has been in place for nearly five years, serviced annually - the last service September 2015, and nothing wrong until this episode (11 and a half months after last service). This has cost me £384 and I'm not happy as I feel my own GSE should have resolved this.
I would have charged £300 less than that!
The boiler burns gas with air. The ratio of gas to air needs careful adjustment to ensure there is sufficient air to reduce CO production but not too much excess air!