Cheers Mariusz… Pleased to hear it is working satisfactorily for you and giving relatively accurate readings. Nice work!! It’s good when a little project comes to a proper conclusion. 
Mike.
The only problem I see with this is you have a modulating boiler. When your boiler fires up for 1 hour from cold your usage will be much higher compared to when it comes on for 1 hour when flow and return temperatures are warm / hot.
As an example, it can take my house 2 hours to heat up from cold, the first twenty minutes the boiler be at max output (15 kwh), after that it reduces to around 5kw for the rest of the heating process. As rooms start to cool and and call for heat again, it will then still come on at low output as the system is still warm.