i would have charged £45 (my min) but i would have said to customer you may as well pay bit more and have the whole lounge done.
if they argue about the min charge for just a stain, tell them you cant make a living out of just stain removal.
we got bills to pay like everybody else.
course supply and demand will play a part, the busier you are the more you'll have to charge, as you dont really want that crap sort of job when you got real good paying ones to carry on with.