Update on this. I purchased a used fan clutch from Mark at Berrys and installed it on the weekend. It's only had a couple of short drives so far but the problem appears solved. The fan is not running full tilt all the time like it was before. It is much quieter and the car seems to have a bit of power back.
I like to learn how things work so I couldn't resist pulling the old clutch apart. The pic below shows the result.
The top half is the bit that is mounted to the water pump. The part in the centre with the 3 round holes is attached to the shaft that mounts to the water pump and therefore spins at the same speed as the pump. The outer section moves independently so I'm guessing that's where the 'clutch' part comes in. The bottom piece is held on with four screws to that outer section and then the fan mounts onto the four studs. You can just make out a clear orangey coloured fluid in the holes. This is the fluid that, I think, changes viscosity as the temperature changes. At the time I took this it was around 20 degrees ambient temp and the fluid had a viscosity similar to molasses.
The rubber seal around the edge of the bottom piece appeared to be in good condition and there was no evidence of fluid leaking out of it. Maybe the fluid loses it's ability to respond to temperature changes over time? No fluid will last forever I suppose.
If I were to try replacing the fluid some more research would be needed to find out how much fluid the unit holds. There are no markings on the outside to indicate the amount of fluid it needs. I guess you could drain and measure then replace with the same amount assuming none has already been lost. That would take some trial and error and possibly risk overheating if there is not enough fluid.
Anyway the problem has been fixed for now so I'm happy.
