About to install a kitchen extractor.
>
If you mean an over-hob extractor these are generally powerful enough to overcome the resistance of most outlet arrangements. Usually they meet the regs on the middle speed anyway.
2. From other posts, street opinion seems to be that centrifugal fans
> out perform axial by a long way - right/wrong?
Not always. There are some good axial fans in very expensive units, but I think price for price you are right.
3. Are the various offerings in rectangular ducting mutually
> interchangeable - or do you have to stick to one brand/make?
Nothing a bit of solvent weld won't fix.
4. Do the corrugations on the flexible round ducting interfere with
> flow enough to make a difference?
>
Yes. Don't leave it all there - cut it to length.