The other day my RR failed its MOT due to one of the rear pads (left outer) being worn down to 1.5mm...all the others were 7/8mm. I completely re-did the rear brakes, changed the discs, replaced the pads, and cleaned the whole lot up. The only other thing i found is that one of the right hand carrier guide pins was fairly well seized... So what do we think happened? does all the pressure go to the LH rear because the circuit senses that the RH is seized. Why would only one pad - the LH outer be worn so badly, while the LH inner ie piston side was hardly worn at all ?

Left hand pins seized. Think about how it works, the inner pad is pushed in by the piston and the outer pad is pushed in by the calliper sliding towards the inside of the car. If it can't slide, the piston releases the inner pad but the calliper doesn't slide so the outer pad is permanently binding.

Sure, but both guide pins on the left were fine. Perhaps there was some other obstruction, or they were choked with brake dust. Anyway, hopefully will all be fine now