As an update, and in contrast to my earlier claims of reliability, we're changing an engine today (ouch). The first indication that all was not well was that the ITT was 80 degrees higher than normal, and higher than the other engine. At first it was thought to be bleed valve failure, but on examination a hole in the engine casing of approx 2.5cm was found just in front of the bleed valve. Theories as to the cause abound locally, the most repeated of which is ingestion of some small foreign object, but I'd be interested to know of other suggestions/experience.