Not having the full service history of my D1, I replaced much of the cooling system as a pre-emptive measure: water pump, fan clutch, hoses, radiator, and thermostat. My local indie Rover shop persuaded me to buy a Nissens aluminum-plastic radiator and since installing it my coolant temperature readings stay in the 189-194 F range. On occasion they'll go a couple degrees higher in certain conditions but drop quickly. Granted, I changed several components at once so I can't isolate the effect of any single part but I also can't complain about the results.
I'm not sure what to expect from this radiator in terms of lifespan. It's been five years of reliable performance so far. I did recently get my hands on an all-aluminum, 4-row radiator that I plan to install at my next scheduled coolant replacement interval.