I'm 99% sure it's the thrust bearing but I'll post my thoughts and just make sure before I pull it apart.

Clutch left alone - rev engine no noise

Put clutch down - rev engine - noise when revs decreasing. VERY sharp and metallic, Grating, like a hammer drill.

Could this be crank bearings? it sounds so damn nasty.

I'd say either clutch release bearing (as it's under pressure) or otherwise spigot bearing as it wouldn't be spinning in the first scenario but is in the second. Fairly easy to change both at the same time.

where do I get a spigot bearing and what does it look like?

CBA wrote:where do I get a spigot bearing and what does it look like?

The Spigot bearing is located in the end of the crankshaft, it locates the end of the shaft in the bell housing, to keep it all in line etc.. When you put a valver in, you need to put the bearing in the end of the crank otherwise you get mucho problemo's.. but anyway, it sounds like its the spigot bearing to me as well, but while you get it all apart to change it, you might as well change the release bearing too, just in case Thomas...........