The mentor box 'problem' only became fully known when actual mentors started to use them for real. The 'fault' which caused the malfunction of the mentor box in the recent airprox was a lose wire, as happens often with headsets etc. given lots of use.
However it has given us (support staff) the ammunition to fire off an urgent Investment Proposal (NATS management speak for getting them to spend some money) to completely re-design and purchase replacement mentor boxes soonest.
BTW for those who find they don't like the background/foreground radar picture it is very simple to change the MMI, making background = foreground, so you can see EVERYTHING on your screen.