I own a leasehold garden flat and want to enforce a nuisance annoyance clause in my lease against my neighbour who is building a shed

The shed is 8 feet tall and 10 feet deep and placed next to our shared boundary fence. This is annoying as it will cast a shadow over my garden and has blocked my view that I've had for 27 years. Is the covenant likely to be enforceable, does the freeholder have a say in what can and can't be done and how expensive is it likely to be if neither party acts on solicitors letters? Also what is the likely fee that a solicitor would charge to write to the neighbour? The neighbour is also a leaseholder and currently has no planning consent for the shed which I believe they also need.