Ann, I think you can just install the relay if the rest of the wiring is correct for a pre-engaged starter. However, don't go by my word alone. Minis stopped being imported here by about 1970 so my exposure to the pre-engaged starter and its wiring is on other, non-Mini cars.
Rusty, if the starter is turning over as you have it wired, it "must" be correct. When two spade terminals are present on a starter solenoid (wing mounted or piggy-backed on a pre-engaged starter), one terminal is for the power "in" from the ignition key circuit. The other terminal is power "out" to the coil when/if a ballast ignition system is fitted. If you had the jumper wire connected to the wrong spade terminal, the starter would simply refuse to turn over.
