MBQ,
"What's to understand? BG specified exactly what was required...."
Could well be that the BG engineer was wrong!
He was when one tried that little 'trick' on me a few years ago, on a two year old wall mounted Baxi Solo fitted into a cupboard (bought from BG and fitted by their installers) with a balanced flue and no wall vents in the room - and also telling me that he was condemning the boiler anyway as it shouldn't have been in a cupboard (despite a bloody great notice [still there] that was stuck on the boiler casing saying the contrary).
BTW, this boiler choice and installation was as discussed and agreed with the rep who called to give me a quote to replace a rather dead Potterton Neat-a-Heat - as I wanted a direct replacement in the same location and no bloody great holes through the wall.
A very short discussion with the 'engineer' (newly employed by BG at the time and on a sales commission) - along with the reading of the boiler spec's soon put him right - and no problems since (even with different engineers).
He just tried it on with the wrong person - and after I had just done a straight 24 hour stint with no sleep on my normal day shift and about six overnight emergency call-outs - boy was I grumpier than usual.
Tanner-'op