I'm going to have new pads fitted at the same time as changing the rotors. Looking at the pad thickness through the wheel spokes, it would appear that they are one half to two thirds of the way through their working life anyway.
I'm using MB parts at the MB dealership. I don't know about a brake fluid change as that was done 4 months and 8,000 miles ago so shouldn't really be necessary. The price I've been quoted isn't bad: GB pounds 170, which is about US$250.
I'm now almost certain the rear nearside (left--my car is rhd) disc/rotor is warped: careful listening shows the sound is coming from that area; the sound goes away on braking and there is a faint vibration in the pedal; and when I pulled away from my office this evening, it was as if the rear nearside brake had become bound on, in the way that the brakes on a car with rear drums bind on when the car is left standing for a long period with the parking brake on (I say "like" this--my car obviously has rear discs and I don't use the parking brake when parking on level ground).
Thank you for your comments.
1999 SL 500
1993 320 TE
1986 500 SEC