Low loss header

[Greenbeast]

Ah i see, from what i read this is unnecessary, certainly in the commercial headers because they have baffles to separate the flow/return water

[Brandon]

The neutraliser will prevent the rads thermosyphoning, as they will not see the pressure difference, and therefore not the motive power for circulation.

I would put the rare on the pipe work of the boiler that you are planning to use as your primary heat source before the neutraliser or LLH.

I have seen several old systems where the two boilers are joined with swept tree, then into the coil, no neutraliser.  If you are purely gravity then that might work.

[Greenbeast]

Ok what about siting the neutraliser on the same floor as the rads (first floor) and plumb them on a circuit off of one set of outputs?
That way the boilers will thermosyphon to it and the rads and the cylinder will thermosyphon from it

I can do a pic if necessary

[Greenbeast]

Perhaps ignore the precise piperuns to the neutraliser, just the overall idea

[Brandon]

look sfine to me, save for the pump on the DHW return, not sure you need that