Are you the first owner / part owner of the house? If so, and especially if you have been in the property less than two years, the work should be under warranty and the contractor should come back and fix it free of charge. I would suggest getting in touch with the housing association (especially if it is affecting other properties), or failing that the developer who built the block of flats, as they should do an investigation of the extent of the problem, rather than you having to guess, and they are likely to have more sway with the contractors, who in my experience are likely to try and bamboozle you, if you are not experienced in house building. Even if you are not under warranty I would contact them first anyway, as this is highly unlikely to comply with the building regulations, and if not the NHBC should not have signed the building off.
Not sure from your post whether you are wondering if it's easy to fix yourself (I could be mis-understanding!) but if so, I would advise against it. From bitter experience I've found that even the seemingly easiest jobs like this in the house are really tricky and stressful.
I wouldn't have thought it would be that difficult for the HA / developer to fix, as it should just entail re-routing the ducting accross the ceiling and cutting the external brickwork so that it can be fitted to a vent leading outside? Might just be more visible ducting between the kitchen and the outside wall.
Not to worry you either, as it might not be an issue, but when you speak to the HA / developer, check about fire stopping measures. Usually with ducts / pathways between different flats / areas of a building, there are fire stopping valves etc to stop a fire travelling easily between the areas. If for example the cooker extractor was designed to go straight outside, there may not be any installed, which could be dangerous if one of the flats in your block were to have a fire.
Out of interest, do you mind me asking who built the block?