I've clayed a few times, check the edge of the clay after you have rubbed part of the car, if your clay is a light colour you can see the dirt starting to build up, you should fold the clay every few wipes.
Try it yourself, its not that hard.....if you have the time that is.
It cant be cheap getting someone in Knightsbridge to detail your car?
2014 F31 328xi Jet Black 6MT
Previous Cars:- 2001 E39 530i > 1995 E34 M5 > 1999 Alpina B8 > 1992 E34 M5 > 1989 E24 635CSi > 1984 E30 323i