If you are not the owner of the house, then legally you can not sale the house, meaning that anything you have signed is not legally binding, which means they shouldnt have accepted a signature from you.
I have just sold my house, within 2 weeks of going on the market. I have not had to pay any up front fees. The £70 you have paid, sounds like the fast track fee, although I would have thought you only pay that once a sale is going through.
I am with a national agent, Bairstow Eves, who have been very fair, professional and honest.
Goodluck, hope it all works out