Land Rover radiators are cross flow - the tubes run horizontal between the tanks on either side.
"Normal" radiators have vertical tubes between a top and bottom tank.
Gravity causes sediment in the water to drop-out of suspension. In a normal radiator the sediment falls into the bottom tank, which is easy to flush out.
Gravity works against cross flow radiators and the sediment drops out of suspension inside the tubes. As this happens the velocity of water flowing in the tube slows, which promotes sediment drop-out until the tube is blocked.
Flushing does not work because there is no water flow in the blocked tubes. The only way to clean the tubes is to remove the tanks from the core (they are soldered on) and physically push the sediment out of the tubes with a rod. Once the tubes have been rodded, they can be chemically cleaned to remove any remaining build-up. Then the tanks are soldered back on and pressure tested.
The above is a job for a radiator specialist (Natrad and the like) who do this sort of thing routinely.
I see the water pump and thermostat have been replaced. The other thing that is often overlooked is the viscous coupling for the fan - they become fault over time and it is surprising how often they are the cause of overheating (even when they look/feel ok).

