So an update to this thread, I had the second brake inspection done and have had it confirmed to me that there is very little wrong with the brakes on my car, front or rear....
The rear discs are virtually new, with the pads about 50% worn, the front discs are more like 50% worn themselves and the pads are new. This corroborates the brake test information sheet I dug out from Mercedes Beaconsfield from December, which shows no faults. Interestingly, the place I took the car to have the tests done, actually pre-ordered a pair of rear discs 'just in case' they needed to fit them (the owner was saying how he nearly had heart failure when he saw the cost
), which I thought was quite the risk for them to take.
But it does show that it would have been in their interest to 'just fit them' (if they were that way inclined) - so i'm impressed by their honesty.
The only thing they did confirm was that my front discs are warped a little, as I have vibration through the steering wheel when slowing from high speed (on light braking only). Given i'll almost certainly have the fronts replaced on the next service (November), i'll just live with that.
So all in all, a win in the respect I don't have to chuck a load more money at the car unexpectedly