The nest thermostat is a combination of time clock and thermostat.
The base “receiver” unit can be fitted next to the boiler where there will be all the cables that you require.
The control or thermostat unit would normally be placed wherever the old thermostat is mounted and only needs 2 cores. This is to supply power from the base unit to the stat unit, as per the instructions.
All the control signals are sent wirelessly from the stat to the base unit.
Although this could be done by someone good diy skills and electrical knowledge, I would recommend getting a professional in to do it.
A mistake in the wiring could do any of the following.
1. Write off the boiler control board
2. Write off the nest thermostat
3. Cause a shock or other injury
4. All of the above at the same time.
As long as you have a wire with at least 2 cores going from the boiler to the old stat, a professional should have you up and running within the hour.
P.s. for the techs reading this, the only function of the link wire from base unit to stat is to supply 12v dc to the stat for its own power.