Up to this point I was still not in posession of a Type-9 five speed gearbox. It seems to be getting harder and harder to find a good example at a fair price (excluding fully re-conditioned units at £400, inc. bell housing).
Finally I won a good Type-9 on ebay for a reasonable price. Problem was, like so many, they are in other parts of the country. A 250 mile round trip was needed to pick this up. Apparently, this unit was bought as a spare by a Robin Hood owner. It was removed by the Robin Hood owner after driving the Sierra home and appeared to be working fine at the time. Fingers crossed, we'll find out if that is the case (much) later on.