Marky,
I've had this problem in my R for the last few months, but not to the extent that you seem to have it. The dealers had my car in to fix other things (which involved removing the wheel, greasing the pads, etc) and ever since that I get a slight clunk when reversing into my drive. I believe it is the front passenger side brake pad moving slightly in the calliper as the car changes direction. I'm told its quite common - and even normal to some degree.
But I will be asking them to look at it in the service which is due soon. Maybe my pads need replacing or some other adjustment is needed.
