Routing cooker hood ducting

[rubble2]

We are about to refit our kitchen. There is an existing cooker hood but this is not ducted to the exterior and is therefore pretty much useless.

To complicate matters there is redundant grill on the outside wall which is blocked off inside and my measurements suggest that this is in line with the interior wall of the kitchen and originally had ducting that ran along the top of the wall units.

The hob is (and will be) positioned such that hood is on an interior wall so the ducting will need to run for about 1.5M parallel with the wall before it can exit to the exterior.

Ideally I would like the ducting to be hidden but don't particularly want to punch another hole in the outside wall essentially next to the existing one so it may have to run along the top of the wall units again and meet up with the existing exit hole.

If I opt for this method, how do I exit the ducting through the side of the extractor chimney? is it simply a case of cutting into the side wall and if so what would be the neatest way of achieving this?

Alternatively is there another way which would enable the ducting to be hidden until it exits on the outside wall?

Appreciate any help on this. Pictures included below.

P.S. I cut out the large hole in the wall to see how much clearance there would be behind.