Attic/loft insulation (yes another)

[ftumsh]

Lo,

I'm planning on insulating my loft. It has some insulation
from when it was built circa 1966. The loft is used for storage
and has a central heating/hot water boiler, along with all the
pipes.
As I'm worried about burst pipes, I thought I'd insulate the roof,
rather than the floor of the loft. Having researched a bit, it
seems that I have to have 50mm clearance between the insulation
and the roof tiles, for ventilation.
There is plastic sheet under the tiles, but I doubt that it is
permeable.
I have vents in my eaves, which currently seem to be covered.
(And have been for 40+ years). I'll also be putting a wood floor
down in the loft before I start. This floor won't extend into the
eaves, just to the where they start ie above the outer wall.

I thought I'd follow this design:
Kingspan Insulation - Self Build

ie the K7 with possibly the K18.
So, for the questions:
1) How far from the apex of the roof should I stop boarding,
to allow for ventilation?
2) Should the eave end of the insulation touch the new floor
or should I leave a gap?
3) Will the insulation have any benefit given the gap in the
apex?

Thanks

John