I've got hold of a service manual, looks like it its the "oil pan upper":
oilPanUpper.png
Agreed, it does look like alloy now that i've leaned it up. I don't know much about the ZD engines, so not entirely sure.
What worrying me is that the removal steps state 'Separate the transmission', not sure if this just means undo the bolts between the oil pan and the tranny, or whether the whole things needs to be separated from the engine. The manual isn't that clear.
oilPanUpper2.png
I'm getting my garage to have a look at it Saturday morning, so hopefully then can confirm how serious this is.