Well.....three hundred or so miles later. One day spent at a local garage getting a replacement fuel pump, and a second day on the trails, I reckon I have a success on my hands with a replacement fan clutch.
Hayden describes three fan clutch performance levels, Standard, Heavy Duty and Severe. Digging through another Rover forum (not dedicated to Defenders), I identified the advanced auto PN 215158 as a severe duty fan clutch, and 215157 as heavy duty. Severe being for the toughest cooling challenge. Since I have a bad habit of overdoing stuff I got the severe duty version. Plus a a Dorman help kit 14040 (four 10mm bolts) and finally a 25/64ths drill bit cos I couldn't find a 10mm version and 3/8s was too small.
I doubt outdoor temps exceeded 70F this weekend. On the road, water temps happily stayed at 180 until I found a few good hills on I91 in VT and intentionally accelerated to stress the cooling system out. It rose up to 195/200 region but dropped back down as soon as the road leveled out. It will idle at 175-180 in a parking lot. Some people say its a louder fan set up. It may be, but no louder than the start up noise you get from the stock unit.
In the back of my mind, I can see having a winter set up, (standard duty) which will cool less and allow more heat output from the heater. This severe version will be better in spring or summer.