Assuming this dispute will end in a legal venue, you need to line up all your ducks in a row.
I suggest you get the ball rolling and write a polite note to the agent referencing the completed job performed, when only a quote (presumably free of charge or for a nominal fee) was to be obtained, and ask the agent “on whose authority was the contractor/tradesman engaged to perform the completed work”? You could, as I do, throw in “please reply within 48 hours or without further correspondence I will refer this matter to NSW Fair Trading”.
Only when I include the above last line do agents ever reply in a timely manner, knowing that I am not pussy footing around.