In addition to Jed's post, I would consider adding to the comunication to the HA, that as far as you can tell, there has been no impact from your work on their property, and the occupier has not informed you of any either.
Whether you want to tell them that you are not happy with their heavy handed attitude and ask if its their policy as a social housing organisation to work with the community or against it, is up to you. ie the little hitler may not be acting in accordance with the associations policy or aims, but just on a power trip.
Bear in mind that there is no claim unless there is damage resulting from your work. There is no penalty for not following the Act.
A housing association would not normally spend money on an injunction unless there is actual or almost certain damage or risk to it. It is just as much risk (perhaps more so) for them to persue legal action, as it is for you to carry on without the PWA, and they tend not to have the money, resources or motivation.
