have just replaced caliper and pads for MOT but near side still registering little effort. tried as directed to adjust caliper slack but to no avail, could it be the cable is stretched and needs replacing

It is difficult to respond without asking you about the basics. A common error that can occur is that the pads have to short pins on the back of them which locate into two recesses on the piston. Unless these are properly located the piston will not be held in place and not allow the adjuster to work properly.
Just a thought.
Regards
Stephen

I suffered the same problem (don't you just hate these Fiat disc set-ups) I had to strip and reassemble mine after being advised that I had adjusted them wrongly, apparently the disc brakes adjust differently to the drum brakes (which I am more familiar with)
You need to make sure you have those 2 stubs on the piston lined up with the pads dimples, then operate the foot brake to push them together to lock them, then the handbrake lever will operate to take up the slack.
It's been a while since I did mine so if I have made an error here will someone correct me.
Thanks
Oletimer
(George)