I am so loathe to get anyone offside as am concerned about the possible further ramifications. My requests have already created unpleasantness, which does make me wonder what is being concealed.
The “unpleasantness” may stem from the other owners having something to hide or it could just be that the fact that you are asking questions makes them think that you believe they are hiding something, which may not be true.
The problem here may simply be lack of transparency, something they should have considered when they embarked on the works you are worried about.
You are right to take the one-on-one approach initially. If nothing else, it will give you more ammunition if and when you head to Fair Trading.