Sometimes it's a case of if you don't ask you don't get it back. I've been in a situation 6 times where I've asked for my money back. 4 times were fine, the other two involved an argument.
A few of the easy ones took place a few minutes into the punt and it's because the girl lied and wasn't providing FK or OWO. I just get up, start dressing and tell her I paid for a service I confirmed and was lied to. Give me my money and I will go. One of those times the money was still on the table so I told her I will be taking it and leaving, she didn't object. Another time I actually asked after half an hour if I could cut the punt short and get a refund on the remaining time as the punt was dire and I had no desire to go for a second round with her, and since she had tried to convince me to stay for only a half hour to begin with, she didn't object and gave me £20 back.
As for the other two, well I reviewed one recently and got my money back, and the second (or first as it was last year) was a Chinese girl in Bayswater. Again, lying about services (FK & OWO not provided) and refused to go for round two. Wanted to spend the rest of the hour massaging me, and even wanted me to tip her
Eventually got half my money back.
It's better to walk if you don't like what you see or the services you initially agreed over the phone suddenly become extras or not on offer. Some lowlifes even lie to your face when reconfirming and therein lies the problem. You can either choose to continue with the punt or try and get your money back. Sometimes you're offered another girl in the flat as a solution. I've always declined that solution.