75k is too much,, and u can't be sure if they really understand the rules,, i tried to hire an agency before to help me with my application but i dismissed them as i know more about the rules than they do.. loads of people in here can help u so don't bother hire someone as it will cost u more..
Did you pay also about your son sole responsibility paper?
sorry but i don't know what u mean with that, sole responsibility is not a paper, it's more of u proving to the ECO that u are the only one who supports ur son financially and emotionally..