It looks like the low speed setting for the fans has gone.
There's more than one possible explanation, so some diagnosis will be necessary.
Take care, the fans are very powerful and dangerous if not treated with respect. Keep anything dangly away from them and don't touch them when they are spinning.
In normal operation both fans should come on at the same speed and at the same time.
Unplug the fans one at a time, apply 12v to the terminals and see if the fan turns.
Fan wiring colours should be
Brown/black - Earth
Red/white - low speed power
Red/black - high speed power.
If the fans fail the low speed test then it is the fan that is faulty. Replace the burned out resistors with external resistors mounted on a cooling plate if you want to save the cost of new fans. I'd recommend this, as the fan resistor is a known weakness and it seems ridiculous to replace a £100-plus fan to fix a 50p burned out resistor.
If the fans run at both low and high speeds then the next thing to test is the thermoswitch. This is mounted in the radiator header and measures the water temp in the rad so the fans only come on when they are needed.
You have plugged the fans back in haven't you?
Take the plug off the thermoswitch and test by applying 12V to the wiring loom plug. You will need the ignition to be turned on, but the engine need not be started.
Wiring colours:
Red - power to the thermoswitch
Red/white - low temperature output (92-97 °C)
Red/yellow - high temperature output (99-105 °C)
So you want to apply 12V from the battery to the red/white and red/yellow connected pins in turn. If the fans work then the thermoswitch is faulty. Replacement is straightforward but it's in an awkward place.
If the fans only work at high speed during this test (but low speed worked when you tested the fan directly) then the fan control module under the battery tray is faulty, or possibly the wiring loom and/or connectors. Inspect the loom and connectors to eliminate them, buzzing them out with a continuity tester if at all possible.
