Hi Paul.
Did you get this sorted yet? I've just today managed to get a (class 4) MOT for my MJ after three test failures for front brake imbalance. In the end I had to replace the hydraulic flexi hoses (although I know there's only one on the back and that won't be causing your problem), both slave cylinders, new linings and a complete strip down and re-grease of pivot points, link-arms etc.
For the class 4, brake imbalance only seems to be a fail point on the front (steered) axle. I guess you're getting an HGV MOT?
Chris