£50 deduction came about out of experience. It used to be a deduction of the price of the clean, trouble was it didn't hurt too much and taken for granted. When you work out the true cost of a callback in time, aggravation, rescheduling, fuel, wear and tear on vehicle and equipment it is surprising. Small things and unfounded complaints don't attract the £50, but genuine problems do.
Now we get quality work, happy incentivised staff and no callbacks.