Robin,
We did turbine washes all the time on helicopters I worked on it was part of the routine maintenance.. When I had a survey done on Nipper the inspector condemned the engine because it had some corrosion, it ran and started fine and all that was needed was a rub down with a wire brush and a lick of paint.
He also highlighted that the toilet sea cock was not accessible without breaking your arm in two, I rectified this by replacing the heads floor and moving the access hatch to allow easy reach. Most surveyors will always air on the side of caution and you need to look for yourself to see if things are actually as bad as he reported.
Doug.