Thanks very much jj4091 & NoPoke. I'll use the uPVC soil pipe as well to improve the flow which will also be easy to cut into if I need to add an inline fan as well.
Does anyone have experience of a practical maximum length of horizontal extraction ducting for a cooker hood?
Mine would need to be 3.7m. Is this likely to work or should I just resign myself to a recirculating filter (would rather go for extraction)?
Thanks.