I spoke to Steve Tuckey at GPR last week about this, his take was they never sell spares as they've had too many problems with bodged kit, but they keep the servicing costs really low (non-profit on the labour) to encourage people to send stuff in.
It's also nice to know that the sames guy working on your kit prepares many of the WRC/GP2/GP Masters/BSB/Le Mans teams brakes. Got to be worth the hassle.
Ali