I've never heard of putting diesel in the rad to clean it!? 
Old coolant does go a bit rank eventually and makes a greasy/shimmery layer on the top, like soap scum around the bath lol. If you had a lot in there it could be oil from either a blown headgasket, or the oil cooler, BUT its rare on the HDi.
And yeah to bleed it run with the rad cap off, open all the bleed points then close each one off as the water runs clear (no air) starting at the lowest one and working upwards.
Old coolant does go a bit rank eventually and makes a greasy/shimmery layer on the top, like soap scum around the bath lol. If you had a lot in there it could be oil from either a blown headgasket, or the oil cooler, BUT its rare on the HDi.
And yeah to bleed it run with the rad cap off, open all the bleed points then close each one off as the water runs clear (no air) starting at the lowest one and working upwards.