Quite simple in the end.
I filled in the V5 ticking the Diesel box, leaving engine number blank.
I included a written note explaining that the diesel had been fitted prior to my ownership and was a LandRover engine of equivalent spec' as the petrol.
V5 returned couple of weeks later duly updated.

I think the thing to do is wait 'til any other changes have been completed so as not to complicate matters.