Try a bit of parallel thinking, the exhaust flue (in the middle) sticks out
furthest to separate the inlet a & exhaust gasses, ok if that can't be done,
think about separating them another way, fabricate (or have fabricated) a
collar that will allow the exhaust pipe to stuck through but will cause the
inlet gasses to be drawn from the side, attach a short duct to this to draw
inlet gas from say a foot away from the exhaust. Then recess both inlet &
exhaust behind separate grilles. Note that such a collar/arrangement is in
common use with many boilers to convert concentric to separate flues,
even if they are not available for your boiler you may be able to source one
for another boiler to use as a starting point.

Thanks Fred, that's a very good one and may yet come in handy if the
next para doesn't work out

More developments on the political front. The council's senior
bureaucrat was ambushed yesterday after lunch, denies all
responsibility, recognises the need to leave modest flues on some old
buildings (no sombrero ban either) because of difficulties of
alternative solutions etc, conclusions: it's all the builder's fault.
So the silly *******s may have to abseil down the building and
reinstall the flues. Do you think I could turn this into a telly series?