we dont get smells at all from the below kitchen
Says the neighbour, or says you?
Yes my wife wasn't happy and said how can he come round telling us to do this and to do that in our kitchen.
Because your regular behaviour is a nuisance to him, as would his predilection for burning tyres in the garden when the wind blows towards your house be a nuisance to you
Fortunately you seem to understand the sense in being neighbourly
my wife said she wouldn't like that.
but she's fine with a larger pipe coming out from the top of the boiler and running under the kitchen ceiling. If the cooker hood had been ducted through the bathroom before you bought the house she wouldn't even have raised it, nor would she have said "I'm not buying that house because it has a low profile duct on the bathroom ceiling
You still haven't posted a floorplan, and this info coming out in dribs and drabs is incredibly tedious, but at least we now know you own the room on the other side of the wall, which makes it feasible to duct up and into the busy corner in the kitchen then go through the wall very near the corner so all there is in the bathroom, if anything, is a bit of duct crossing through in the corner of the room. That may then be possible to hide with coving, a corner shelf etc
Alternatively, if you
post a floorplan with joist directions annotated we will be able to see if it's possible to go through the joists above the bathroom because they're clearly shorter and don't need to be as strong
Post a floorplan with neighbouring rooms so we can see what is on the other side of the wall furthest away from the cooker
You clearly know how to post a photo so get a pen, draw some hasty rectangles on a piece of paper repeating a floorplan and take a photo of it and post the photo