Thank you for the information and the pictures @minos_GR . For starters you can have a look here: https://bit.ly/2X5uDbn . This is our installation video for the Google Nest E so you can get an idea of the whole process.
Based on the diagram it would seem that your current thermostat uses a live, neutral and a call for heat wire.
In this case you would need to cap off the neutral wire. Use the live wire in terminal C on the Heat Link and the call for heat wire in terminal N0.
Based on the diagram in the cover it would seem the neutral is in the L4 terminal and the live in the Y2/Y1 area. The additional 2 2 wires probably go to the boiler.
However, we can't say this for sure, as the diagram only shows 3 terminals but there are more connections and wires than that.
You can get in touch with a regular installer, if a Nest Pro is not available in that area. As long as they can determine which wire is which they should be able to connect it as they only need to identify the live wire and the one that goes to the boiler.
