Just returning to this.
One MOT failure owing to pads (meat left) and an advisory on the disc thickness.
The current distance between the pads is a shade over 25mm at centre point with the verniers and there is at least 3mm left on the pad with the most wear, which has not started any grinding noises in the 2000 miles or so since the last MOT. Others are 4 to 7mm from top to bottom.
I have had no reasons to think the brakes are deficient but will replace the pads anyway, leaving the discs for another time but the advisory is 'replace as soon as possible'.
It may be an over eager tester, unless there are more stringent rules in place that I don't know about.