Testing a co detector was not part of the annual service visit as far as I remember.
A leak from a combustion box is serious but to give you further info the make and model would help as seals etc can be replaced if available.If however the leak is from a rusted hole in combustion box then depending on age/make/model you may be looking at a new boiler.Come back.
Flamingo 2 or Z RS40, sorry can not read engineers writing on 2/Z