Ask an Property Solicitor. Get an Answer ASAP.
How long has the previous owners flue been in place for?
Why has no application be made to remove it?
Would they be attaching the new flue in replace of the old one or in addition?
You say that John owns the flat. Who owns the freehold who would normally be responsible for the external walls?
Why do you think that they should have asked John’s permission?
What is it that you don’t want happening again?
Why don’t you want it happening again?
Thank you. They would not need to seek permission from the flat leaseholder, but they would need to seek permission from the freeholder that owns the building.
If this is unsightly, there is probably little that you could do about it. If however it produces smells which come into the property or it produces noise, then you have a claim against the restaurant in nuisance as an occupier and so does John as the owner.
It’s essential that you object in writing to the planning application and if there is any hearing, that you attend. If you don’t attend, it’s likely that your objection will not be taken that seriously.
You cannot object simply because you don’t like it, you have to object because of the adverse effect on your property which what you’ve done in your numbered list.
If you use a planning consultant to formulate your objection, it will be worded in the most powerful way but you would be forced with paying the costs of that and could not recover it.