My boiler is currently powered by a switched fused spur. The spur is located 40cm underneath the boiler.
I haven't checked but it is probably a 3A fuse.
My boiler is

a Ideal Logic combi ESP1 35 : idealheating.com/.../logic-combi-esp1-installation-and-servicing.pdf

I would like to add a Nest thermostat using the opentherm wiring as described here page 24: https://nest.com/support/images/misc-nest-thermostat-eu/gen3-install/3rd-gen-Nest-Learning-Thermostat-Install-Guide-UK.pdf

The wiring is relatively easy, I just need 230V AC  N + L to power the device and then 2 low voltage wires to the Opentherm on the boiler.

My question is, where do I get the the 230V AC  N + L from ? the easiest way and following the regulation.

Can I get it from the current switched fused spur that powers the boiler ?
if yes, shall I connect the nest heatlink N+L directly from the switched fused spur (load side)   

or 

shall I connect the nest heatlink N+L  directly from the boiler on the MAIN input?

Thanks in advance.