Kevin wrote:

One of the main reasons for using a cooker hood is to avoid a build-up
of steam in the kitchen. For this reason alone, I don't believe
using the hood in recirculation mode would be effective in any way.

Dead right.

snip ducting routing problem

IMHO it's very hard to give an accurate answer, as it's quite
subjective. Yes, you'll get lower flow if you introduce a bend;
introduce another bend; reduce the CSA of the duct; have multiple
changes in duct section. The question is, will you still have what you
will consider to be adequate flow in your proposed setup? I don't know!

Finally, where can I buy a decent length of 150mm round pipe? Screwfix
only seem to do 350mm lengths of 120mm and I would assume that each
connection itself has some impact upon the airflow, or am I wrong?

Have you tried http://www.bes.ltd.uk?

David