Your boiler probably has a setting which adjusts the heat input for the central heating. Has that been checked?
There could also be a problem with the gas pressure at the boiler input.
I have had a look at the boiler details. It is a new design and has some slightly unusual features.
It does not have a maximum power setting adjustment. So it can deliver between 6 kW and 27 kW of heating power at a flow temperature of 50 C. it automatically adjusts the power required to reach the flow temperature that you set yourself on the front knob.
The temperature control knob can set the output temperature right up to 80 C. But the efficiency goes down a lot if you set it above 70 C as it will not condense and thus lose about 12% efficiency.
The latest standard for sizing radiators is based on a flow temperature of 50 C as that is as high as heat pumps might give. At a flow temperature of 70 C the heat output will be higher.
Radiators will always be hotter at the top as the hotter water is less dense. That is totally normal. Lower down they will be slightly cooler depending on how they have been balanced. With a condensing boiler it is best to set them for a differential of about 15 C to 20 C as that encourages more condensing and thus higher efficiency.
Balancing is quite time consuming and is best learnt by doing it. I know a few tricks by first presetting typical lockshield positions according to the radiator size and distance from the boiler. Sometimes I can reach acceptable balance just by that. But that assumes all the lockshields are the same design and the pipework correctly sized.