Owning your own home

Share of freehold with flats upstairs and downstairs. We all have a (private) chunk of the garden but access (for us and upstairs) involves walking across the garden belonging to the downstairs flat (there's no problem with this access, lease allows for it, it's worked well with current downstairs neighbours for the last 10 years we've been here).

Downstairs are selling and prospective buyers have a "small dog". Current lease doesn't allow pets but says that consent should not be "unreasonably withheld". Fair enough, it's not a big enough issue to warrant the sale falling through.

Anyone had similar? What needs considering? Currently on the list:-

• New downstairs neighbours would have to install gates to prevent the dog from getting into the bits of the garden for us and the flat above us (upstairs have a baby and want to use their garden this summer). This would be relatively simple given the layout and existing fencing.
  
• Noise? Can we state anything that could help us if the dog turns out to stay up all night barking?
  
• Limits? What if they want to get another dog? An army of great danes? Only replace it with a cat if the dog dies?