E5 Error on Remeha Avanta Plus boiler?

We have a Remeha Avanta Plus boiler that is about 5 years old. It has started flashing an E5 error. I have had an engineer out 3 times and he has serviced the boiler, and replaced the part according to the book that refers to the error:

Description - No ionisation or ionisation failure more than 5 times during one heat demand.

Possible cause - CO2 wrongly adjusted

Check / solution -
Check:
• CO2 adjustment on the gas block;
• ignition/ionisation pin;
• flue/air-supply connections;
• gas flow at full load;
• if there is no recirculation of flue gasses (either inside or outside the boiler).

I have been pressing the R button to reset it, which works for a couple of days, but then it is back to flashing the error again.

I am at the end of my tether, as the house is freezing when I come down and with children it is no longer funny anymore. Can anyone help please or advise me on what I should/could do?