retest failed for a third time, ££460 down plus mot fee, brake pads and discs, changed twice now in a week, both callipers changed, both side hand brake cable changed and carrier changed (used carrier) and parking brake reading still giving a low reading of below 100 on the right offside.
Everything changed and you still have handbrake imbalance? The handbrake doesn't have to be fantastic to pass an MOT, just even. It should be possible to pass with all new components, before bedding in. Not wanting to sound too critical of your mechanic, but there's something hasn't been done right somewhere. Assuming the service brake results are good, and showing no imbalance, the problem must be with the actuation of the handbrake.
I would be slacking the cables off, reset the caliper handbrake adjusters with the footbrake, then readjust the handbrake cables. This is the correct procedure, if it's not been done this way, you can end up with the actuator on one side not fully retracting, and this does not allow the caliper's internal automatic adjuster to work, giving a weak handbrake on one side.
Observing the cable adjuster and caliper handbrake actuators whilst pulling the handbrake on can give you a pointer - if one actuator rotates noticeably more that the other side, that caliper hasn't adjusted internally, and the cable adjuster will pull over to that side.
Some people fit new rear calipers thinking that they will slot straight in and work - you may get away with it, but you may not... the correct procedure is as above.
There can't be many other options left.