Job's a good un!
The 'D' bit worked just fine. I had my doubts because it seems to lack anything resembling a cutting edge, but what do I know.
The step drilling marks are still visible so I didn't get that quite right. I skipped the filing stage because the thing was set up and aligned on the lathe and I thought it would be easier to just see if the 'D' bit would do the job without any filing (it did).
Thanks again to everyone who offered advice on this. I got there in the end.