Re the yellow flame Andrew, yes, that is what I would expect, but my hearth has a 4psi air blast and although it produced a blue flame, it seems there is a cold centre to the flame, as if the gas is fed to the nozzle round the outside, and the air in the centre. If you look at the nozzle, this seems to be the case.. It was fairly belching out CO, and only took a few minutes to set the detector off, whereas now, with a 37mbar regulator the flame is much hotter, and no CO is detected! It had me scratching my head because as you say, it doesnt make a lot of sense, but it seems to work very well now.
Phil.