Sorry you feel you had such bad advice Stitch.
Was she a specialist family lawyer/member of Resolution and did she have a legal aid franchise (unfortunately not many firms still have them now)
It should have been made clear to you at the start (when you made the appointment) whether a charge would be involved and if so, what it would be. If this was not the case, then you may have a case to argue it. Also, if you made it clear that you wanted to find out about legal aid and that firm didn't have a public funding (legal aid) franchise that should have been made clear.
There are still a few firms around that offer a free one off interview and some that do a discounted rate which may be what you are being charged as to be honest, £75 is very cheap for an hours advice. I used to charge £150 an hour 3 years ago (yes I know that is disgustingly expensive and no, I didn't take home anywhere near that amount ph) but that was in rural Surrey where everything was so expensive. Sorry you've had such a bad experience though.
I hope you manage to find a good family lawyer with a public funding franchise who can help you through this difficult time.