Don't feel guilty about charging makingdo. At the end of the day people choose whether they want the reading or not. I would definatly set a deadline on the free readings otherwise its going to spiral out of control.
Five pound is excellent value for what you are offering, everyone seems really pleased with their readings (eagerly anticipating mine).
Why don't you offer refunds if people are unsatisfied with the reading rather than deciding wether to pay after the reading (less chance of people taking advantage then)?
(sorry for my spelling, very tired atm)