I'm having a replacement CH boiler (Worcester Bosch Greenstar 24Ri)
fitted in a couple of weeks in the airing cupboard in the first floor
bathroom. The existing boiler is immediately below on the ground
floor, so all of the existing pipework is quite convenient.

I've agreed with the plumber that I will provide an FCU in the airing
cupboard, and put in a new cable from the room thermostat on the
ground floor. I think I have 3 options for providing the power to the
FCU.

1. A short spur from the first floor ring main. This is the most
obvious and probably easiest, however I'm not sure about the integrity
of that ring main (it's been extended in various ways over the years,
including to a second floor loft conversion) plus the cable is buried
in the walls in many places (17th regs, RCDs and new CU - shudder!).

2. A short spur from the supply to the immersion heater in the HW
cylinder (also in the airing cupboard). Simple, plus the cable is not
buried at any point (runs down via a ground floor cupboard into the
underfloor void and from there to a cupboard containing the CU), so no
RCD needed. The heater is on a dedicated radial via a 15a MCB. However
the heater is rated at 3Kw, plus say 3a for the boiler, so pushing the
MCB a bit (maybe). Pretty sure it's on 2.5 T&E but need to check.

3. Either extend the existing spur/FCU from the old boiler up to the
airing cupboard directly above or put a new spur from the ground floor
ring. Fairly easy to do. However the existing FCU is a spur on the
ground floor ring main (separate MCBs for ground and first floor
rings) and this ring main is also hidden behind some wall paneling in
the kitchen (elsewhere is just in the under floor void). So maybe
another 17th regs and RCD issue again.

Obviously if the boiler is anywhere other than on the first floor ring
the FCU will be labeled accordingly.

The main thing I'm trying to avoid (without being unsafe - as distinct
from obeying all the "rules") is the plumber's sparky trying to insist
on a CU upgrade to 17th standard, plus all the other bits associated
with this.

What's the "best" option for the boiler supply, or maybe a variation
on the above?

Any constructive advice gratefully received, and any other points to
watch out for that the sparky might be tempted to wring extra business
out of, and where I can do something to head this off.

Thanks, David