CO2 Monitors Yes John there is a lot of detial on it on the Covid guidance section of the Council website, inclduing now recommendations of good CO2 meters to use.
I have done quite a few tests myself which generally accord with what I would have expect - well ventilated ringing chambers not being a problem, and poorly ventilated ringing chambers being a problem.
I have had quite a bit of feedback from others as well, for instance from AJB last week, including from towers where the CO2 readings have been very high. What may be happening in response to very high readings is ringers deciding that the meter must be wrong, or that it doesn't really matter!
At one tower near me, Selly Oak, which is a small and unventilated ringing chamber, the reading went up to 1000 in one burst of wedding ringing.