Plan view so question is blue position or red position.  Note that there are also 2 ovens in tge middle of the back run, more or less lined up with the red position.

Difficult one, but I think I'd go closer to the hob

Thanks Sarah L, I think we will put it as close to beam as possible (although it can't go right next to beam due to the ducting from its own motor etc).

We're also going to move the hob over slightly so it is more in line with it extractor and not so directly under beam as I am having nightmares about condensation gathering on the underside of my beautifully painted beam.  To be honest we are realising at this very late stage that we have made a huge error in judgement and should have just designed the whole thing differently (frustrating as my i initial ideas would have worked but husband wasnt keen on the look so the plan got scrapped) but sadly it feels too late now to change it as it is all insulated, boarded, plastered and painted so retro fitting extra ducting would be an absolute pain and costly.  We will live with this mistake for now as we're desperate to get the kitchen in after 3 months without a kitchen.  We have electrics above the hob for a pendant light which we will check with the electrician if this is also suitable for a recirculating fan in future if we feel it is needed.

Thanks for your input.

Thanks Jonathan, there's a hood chance you're right. Sadly it is too late to change the kitchen design now as it is delivered on Tuesday.  The island units won't accomodate a downdraft.  I did try to convince my other half earlier in the design process but it was a no go and we are passed tge point of no return on that one.  We may install a recirculating hanging hood above an offset hob.  I don't know if we can return the ceiling extractor as was delivered ages ago and has been unboxed.  This has been the most stressful part of our whole house renovation and is causing me to feel quite sick with stress.  I can't wait to be out the other side but realising your mistakes is a bitter pill to swallow.  Our stubbornness has averted several mishaps with our builders and sticking to our design odeas even when they were questioned has paid off for many aspects.  But on this we have been stubborn and now we suffer.

Hi again Jonathan, we have been discussing this not only for months but also all afternoon.  Your wise words have been considered.  My favoured option now would be to move the hob over so it is offset, not centrally below the beam.  Then install a hanging style recirculation hood with an additional vented extractor (which would need to go in the ceiling to join the ducting and electrics already in place).

With these ideas in mind, would you happen to know if electrics currently installed for a metal pendant light would be likely to be appropriate for a recirculating hob instead or would we need wiring stripped out and replaced?

Also we can install a inline vent but then need a separate extractor motor 60L rate which would fit in the 23cm ceiling void.....any recommendations on this? 

Thanks as always for your experienced input.

I think you are going out of your way to botch this. You should stick with your original plan to have the hob in the middle of the island and change the hob for a Cooktop extractor.
Your kitchen installer should be able to adapt the cabinet that is under the hob, or make the cabinet on the opposite side of the island less deep thereby creating a gap to accommodate the venting for a cooktop extractor, or if there is no cabinet there then they can put create a gap between the cabinets and the panel you will be putting on the back of the island.

Thanks Jonathan, I appreciate your frankness.  We are settled on a plan now.  We might do it differently if we could turn back time but we can only move forward from this point.  We are going to move the hob over slightly so that some rings won't be directly under the beam and it will be more in line with the ceiling extractor.  The extraction rate on this is almost 3 x the requirement for building regs away from hob so we are putting our faith in it.

Having moved the hob over so it is only over one drawer stack instead of spanning two, it will be easier to retrofit downdraft or indeed recirculating hood above in future if needed.