Why not go for a downdraft extractor fan and avoid worrying about an overhead one altogether? Then when its not in use it just sinks down into the counter so you hardly see it at all! They can be expensive but theres more cheaper ones becoming available now:

http://www.tapwarehouse.com/product/90-down-draft-hood?gclid=Cj0KEQjwhbzABRDHw_i4q6fXoLIBEiQANZKGW51yCspfNh5KB2yuRlMmzimwvdGFvW91bH6Ul73gCjMaAunw8P8HAQ

Here's a rundown of popular ones:

http://qosy.co/7-best-downdraft-extractor-fans/

Thanks for replying Lauren. I don't want anything interrupting the stainless steel worktop surface apart from the hob itself - plus I've been told they don't always work well at capturing the steam (and many are not suitable for use with a gas hob).

Thanks for the replies so far. We definitely do need extraction as BC are involved. Any more input will be gratefully recieved.

OnePlan what made you select these two fans to show me over the many others out there? I wondered if you have personal experience with either of these fans or the brands?

we went for a Westin ceiling one, same as you, big space, BC, peninsula with hob on. It was not cheap but we couldn't risk moisture due to mould allergy in the family. Our builder & kitchen fitter were very impressed with it and they've fitted loads. Your extraction run out will be the key factor. Ours was a long straight run (Westins can do straight lines or one bend). I'd call them if I were you - they were very very helpful before we purchased. Will try and send you a pic.

I would take a look at Gutman extractors they have a really good range of ceiling mounted extractors with ample extraction rate to satisfy your builder and architect. They also have the option of external or inline motors but that ups the costs considerably. They have a range of ceiling extractors that are quite slim so the ceiling box can be less that 20cm deep.

Gutman Claro ceiling extractor has an extraction rate of 950 m3/h with a maximum of 66db. Really well made product and good value.