Thank you. This is not a matter for the council. Doesn’t bear fruit, make a complaint to the Local Government Ombudsman but being brutally honest with you, I don’t think that either of those would produce a result for you.
Here is the link to the Ombudsman fortunately, it’s free: https://www.lgo.org.uk/make-a-complaint/how-to-complain
It is trespass because whilst he is entitled to build up as far as he likes and could build a skyscraper block (you would potentially have an action against him for blocking your light but that’s a different issue) he has no right to attach in any part of the building which is within your lease and you say that includes the structure and therefore, the flashing between the buildings where as set it into your building is without consent and its therefore trespass.
It is not the height which is the problem but the fact that he has joined into your building.
Because you have allowed this to go on, it is unlikely that a court would order it to be removed but what it is more likely to do is order the owner of the downstairs property to pay you compensation in respect of the trespass and expose the air brick completely not because of the building passing the air brick but because of the potential claiming negligence by the removal of the ventilation from the property.
Check your house insurance to see if you have legal expenses cover that would pay the costs of dealing with this
Can I clarify anything else for you? I’m happy to telephone you if you still want to speak.
I’m happy to answer any specific points arising from this.
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If you still need any point clarifying, I will still reply because the thread does not close.