....and having just had a brief look at the following video, dazzlenation...i think that i need a second opinion....for example, none of the plumbers sent by CHP/aaron services ltd...go near to the control panel...they simply use the readings upon their own gadget....in short, they do not put it into 'full load'....go figure:

How to check CO2 readings - YouTube

oh and the following looks like professional equipment....inserted into the correct etc etc....how unlike some of those so-called gas-safety-inspectors...who sometimes used a long 'metal ruler' to then jab it around within the inside of the boiler....isn't this all so pathetic....one has to trust 'whoever' to come round and do a professional job etc etc......additionally, i might add that the latest 'lee jackson' to do the annual gas safety inspection...didn't bother to do the following....upon both of his visits....last year's, as well the latest gas inspection....he didn't bother to do the following check, at all: