If one is trying to maximise thickness of breathable 'wood fibre' insulation between rafters -ie full depth- is a small 'gap' left between each timber sarking (roof) board sufficient ventilation in lieu of a vented gap above the insulation? If so what sort of gap is required between the boards? Is this approach/detail acceptable to Building Control...I have heard it referred to as 'Scottish Practice'..!

Related to the above…is a gap above 'wood fibre' insulation actually required if one uses a breather membrane such as 'Pro Clima' atop the roof boarding in a full roof retrofit? Context: Traditional building with Planning restriction so cannot add more insulation atop rafters and need to maximise headroom below.

I would say that full depth insulation between rafters is more commonly seen in "Warm Roof" build ups, where there is an equal or greater thickness of insulation over the top of the rafters than in between them,
(and by top I mean the outside bit, not the underside that would be the ceiling to the room)
e.g. 100mm rafters with 100mm PIR between, then 100mm PIR over the top, fixed through to the rafters with counterbattens, then your breather membrane, then tile battens (if tiled roof).

As opposed to the much more common situation (especially in retrofit) where the insulation is between and under the rafters, so 100mm rafters would only have 50mm PIR between with a 50mm air gap maintained at the top between the insulation and the roof membrane (or "roofing felt" if traditional) to ventilate condensation.
then with additional insulation (e.g.150mm PIR) on the underside of the rafters i.e. the ceiling to the room.

in a warm roof, the rafters are kept warm, in a cold roof the top of the rafters are cold but kept ventilated.

I am not an expert and only have experience of warm roof on a recent barn conversion and cold roof on original farmhouse.

Maximising insulation depth on an existing roof has considerations then via both routes, increase in external roof plane height, or loss of internal ceiling height, but in both cases its very important to consider condensation risk.

If a new roof is being built, then warm roof has always made more sense to me.

For a bit more background, the building is a traditional form with 150mm rafters in a sensitive Planning context so need to maintain the roof profile as is ie only insulate between the rafters due to the latter with 150mm wood fibre and maximise headroom within. So my thought / question remains: would timber 'planks/boards' (called sarking in Scotland) with a small gap between -clad atop with a Pro Clima breather membrane- suffice with full fill insulation in lieu of a 50mm vented gap atop 100mm wood fibre and thereby worse U-value.

I'm sure this has come up previously, as I vaguely recall commenting on this, since I've been designing and building roofs with sarking and full fill insulation for at least 12 years. That is in Scotland, but the regs tend to be mostly identical, or slightly stricter in Scotland, due to our more challenging climate.

Definition of warm roof versus cold roof has been picked up previously, but this link to a particular product, shows the roof build ups that they certify. AIUI, highly breathable membranes (that's not every roof VPM) can be substituted for this particular brand.

https://files.proctorgroup.com/assets/installation-guides/proctors-roofshield-installation-guide.pdf

Just check about the requirements for VCL - not sure what you're planning. I think there may be a need to control vapour levels passing through the roof void. I always use a VCL for airtightness, but check that aspect.

Update: Building Control say sarking detail with gaps between boards (in lieu 50mm gap above insulation) would require membrane BBA Cert allowing such-fair enough. That being the case have negotiated/had assessed with Steico to have-natural slate on breather membrane on 18mm WBP ply on 25x50 batten on a further breather membrane on the rafters with full fill wood fibre ie 25mm / not 50mm vent gap .