With much experience of urban living, leasehold property and nuisances, and some experience as a dog owner myself in a flat:
Having established that the head Lease includes a covenant not to keep a dog, then if you prefer not to be disturbed by a dog barking, you really SHOULD simply insist on performance of that covenent, and you should not have difficulty doing so
Any attempt at half measures, in the hope that the dog ceases to bother you, and/or because you don't want to upset people would be a mistake, as if you or the Freeholder don't exercise your rights, they can become weakened.
One thing that often happens by the way is that when a Leaseholder sub-lets the property, they fail to include important terms from the Lease in the subtenancy agreement. They sometimes even (through ignorance rather than malice) write terms which contradict the head Lease.
Of course the head Lease takes precedence, but this can on occasion put subtenants in an embarrassing position. But even if that's happened here (as I say, unlikley), none of that is YOUR problem.
Two caveats however before proceeding:
- You need to be sure or reasonably sure that the head Lease of the property is identical to the one you have seen (presumably your own). Although uncommon, it is by no means unheard of for variations to creep in, even if all the properties were built and initially let at the same time.
- You should read the Lease carefully to understand, for the relevant sections, where the power of enforceability lies. Probably ONLY with the Freeholder, but in newer leases that power may lie with other lessees, such as yourself. (This will be relevant if the Freeholder is sluggish in dealing with it).
In any case, you should write, formally, to the Freeholder, citing the relevant section of the Lease, stating clearly why you believe there is a dog there, and giving a reasonable time by which you expect the dog to be gone. Best to keep this time short, harsh though that may sound.
The key here is to kep this formal and impersonal. This is not about whether dogs can be trained quietly, or to be quiet, whether they should be kept in flats, whether one breed is less suitable than another, or being nice to our neighbours.
The Lease says no dogs, and you either choose to insist on that, or to turn a blind eye, and put up with the consequences whatever they turn out to be.
Good luck