i take it you have done a full service and checked gas pressures? bear in mind this is a cast iron heat exchanger on this boiler if you have cleaned the heat exchanger and flue ways,taken the pressures and the flame picture is ok and it passes spillage and flue flow would not worry to much
When I done my CPA training they never mentioned cast iron heat exchangers. I actually had to analyse an old openflued boiler with cast iorn heat exchanger in my assesment?!
Would love to get clarification on this as I would be a bit concerned if I got those sort of readings as well!