At the very least the landlord or the agent has to give you written notice of entry - which clearly didn't happen with the gardener coming over and just ripping out trees. It's not up to the gardener to call (although that would be nice as a courtesy) - the landlord cannot just get people to come over whenever he/she feels like it without letting you know at least 24 hours beforehand in writing, unless it is an emergency. So they have broken that rule to begin with. Personally I think that building a pool will interfere with your "quiet enjoyment" of the property, so you would be within your rights to submit a written letter outlining your concerns and the predicted impact on your family of the building of the pool, not to mention the fact that when you moved in and signed the lease, it was for a house without a pool - so you shouldn't now have to be responsible for maintaining a pool that didn't exist when you moved in.