Our (group owned) aircraft was found (during the first inspection by our new maintenance company) to have laminar corrosion separating the top cap from the main spar a few years back and the extent of the corrosion was such that the aircraft had gone through approx. two years worth of Annuals, 50's, 150's and a CofA. It cost £12,000 to put right.
The photographs were shown to 3 x CAA Surveyors who all agreed (off the record, of course) that there were very good grounds for negligence on the part of the previous maintenance company.
But we were also told that, even if we did formally complain the chances were that no action would be taken as negligence would be very difficult if not impossible to prove.