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 ?

It's usual to power boiler controls from the boiler supply circuit - and the nest's instructions seem to suggest doing just that - so yes that seems to be the way to go. Regulation wise you can connect it where ever is convenient (FCU or boiler or some wiring centre in between) - but also check the boiler instructions - some prefer controls to be wired after the boiler's own on/off switch (so that that switch isolates everything).

   - Andy.

Thank you, regarding the wire section what do you suggest? 1.5mm2 x 2 (no earth needed apparently if I don’t the t1/t2 on the nest.)

I have an identical boiler (the 30kW version) controlled by a Honeywell Home wireless programmer/thermostat.  As you suggest and Andy confirms, both the boiler and thermostat control unit are powered from the fused isolator switch.  Switching off at the switch isolates both the boiler and the control unit.

David

1.5mm2 x 2 (no earth needed apparently if I don’t the t1/t2 on the nest.)

Depends a bit on your chosen wiring system - typically 1.0mm² solid core or 0.75mm² flex - 1.5mm² solid core is a bit stiff to terminate into many PCB type terminals. It's usual to include an earth (c.p.c.) to all points and accessories even if it's not needed initially - to allow for a future replacement requiring an earth (actually a regs requirement in most situations).

   - Andy.

Thank you Andy, 0.75mm2 2+earth flex for the power, what about the cable for the low voltage  opentherm connection? Any regulations on that ? Can i use the same  0.75mm2 2+earth flex and not using the earth obviously ? Does the wire colour matter from a regulation perspective ?

In general there is no harm in using a mains rated cable on a lower voltage. It's the other way about we worry about !! 

Colours wise, the only one folk get really hot under the collar about is using green/yellow for something that is not an earth, It is however good practice to label the cables - and that could be as simple as writing on it in sharpie, or on a little flag of white tape, just so that anyone coming in after you can work out what is going on and does not have any expensive mistakes.

Mike.