Your pump is too small for heat exchanger resistance plus rads ...
It's only too small if the system has been balanced incorrectly.
The boiler is designed to work with a flow/return differential of up to 20C. The chart below shows the resistance through the hex for a 21 kW output when the differential is 20C (green) and 11C (red). You will see that the resistance of the heat exchanger is proportional to flow rate squared, i.e doubling the flow rate increases the resistance four times.
At the flow rate required for an 11C drop at 21kW output the pump is not even capable of providing the head to overcome the resistance of the heat exchanger, ignoring the rad circuit. The boiler with therefore modulate down until the output of the boiler matches that of the radiators, which will be less than 21 kW.
Increasing the differential, which reduces the required flow rate and the resistance of the hex, should leave sufficient pump head for the rad circuit.
With a flow of 55C and return of 42C (13C lower as per first post) the rads will be producing about 46% of their rated output. So the 19 kW of rads are only producing 8.75 kW. This means the flow rate is 580 litres/hr, so the hex resistance is 0.5m. At this flow rate the pump is delivering a head of approx 4.5m. But this does not mean that the resistance of the rad circuit is 4.5 - 0.5 = 4 metres. It's just that the working point on the selected pump curve (speed 3) gives a head of 4.5 m.
Set the pump to a lower speed (2 as a start) and check the flow return difference. Provided it is not over 20C, rebalance the rads. A good starting point is to set all lockshield valves to half a turn open. Then make small adjustments (1/12th turn) to correct for any rad which are hotter (close) or colder (open) than the average. Keep a watch on the temperature difference at the boiler.
