If there's a brake imbalance measurable then it makes sense that under extreme point of locking braking, especially on slippery surfaces, that whether you can feel it through the steering or not, that imbalance could possibly lead to instability, think how little grip there is on compacted snow for example and how little effort is required to lock a wheel, ABS being little short of useless in such circumstances.

If it had been my car it would have been dismantled on my drive immediately coming home from the MOT test, whatever the cause hasn't mended itself during the year, that slightly sticking caliper slider (if thats the problem) might have seized fully by now and you might not be braking at all on one pad, which isn't the end of the world in itself but if a sliding caliper isn't sliding then something is bending instead, how many braking operations before metal fatigue becomes a problem?

Edited by gordonbennet on 25/10/2016 at 17:58

So I have not ignored the problem as the car does not pull/dive under heavy braking and the ABS works ok as I have tried that deliberately when safe to do so.

An MoT station uses very expensive and calibrated equipment to test the brakes. However, despite their equipment, the testers knowledge and their advice that attention was required, you did nothing except for a bit of hard braking somewhere.

Did you know it is a requirement of your insurance that you maintain your car in a safe and roadworthy condition and that failure to do so can render your insurance invalid? And that anyone can look at your MoT history? If you had been involved in an accident a third party could have used your lack of maintenance as proof of culpability.

"with power comes responsibility"