I am looking to install an extractor fan in a newly created internal
kitchen. The problem I have is that the nearest point to which to extract
(i.e. external wall) is some 4m away.

I will be fitting a 6" extractor as per Building Control instructions. The
ducting, together with the waste from the sink, will run behind a new
partition wall in the adjacent room. I would like to keep this new wall as
close as possible to the existing wall as the room is quite narrow already.

I intend to use a standard wall mounted fan - something like this one:
http://www.screwfix.com/app/sfd/cat/...23262&id=24840).
My question is: will it be OK to run 4" ducting for it along the 4m span?
Can I use something like this:
http://www.screwfix.com/app/sfd/cat/...23473&id=14118 ?
It would make my life much easier as I will only need two flat-to-round
adaptors on either end, and I would lose less of the room. It seem much
harder to find adaptors, etc for the 6" flat one (
http://www.screwfix.com/app/sfd/cat/...23473&id=59769).

Considering that these fan are only designed to work to their full capacity
with straight ducting of 2-3m according to the manufacturer (mine will be a
pretty messy 4-5m), I just thought that I could get away with using smaller
ducting. Is the only drawback the drop in performance, or is there anything
else I need to consider?

I should probably add that I am not too concerned with the performance of
the fan, as it is only there to satisfy the BCO demands, and I will happily
live without the fan altogether (personal choice, I know).

Many thanks in advance for your thoughts.

J.