The problem has got 'worse' over the last few years. I have probably only used them 3/4 times as an in extremis solution (or when a late night curry/bagel is needed). To date, I've had no problems and normally manage to haggle a good-ish fare.

Given the general scarity of black cabs late at night, it's either a 'minicab' or the Fright bus. Neither are that appealing. But that is the state of public transport in London today, sadly.