Just about to rebuild my 1600 x flow and the new piston rings arrived today. I was expecting the oil control ring to be in 3 parts - two rings with a spacer sandwiched between them. What I've got is one solid ring with a circular spring inside it. Is this right?

Think mine was the same , just a different type from what you expected.

Yes this is correct. A cast iron oil control ring with expander spring is very common.