Johan wrote

I know this subject has been covered in various ways

You're not wrong there! LOL

This is what I understand (correct me if I am wrong):

In a warm roof construction, the insulation material is above the
rafters and continuous, and ventilation is not needed. On the warm side
of the insulation a vapour barier is needed.

In a cold roof, where the insulation is for example between the rafters,
you need to keep 50mm air space between the top of the insulation and
the roof covering (felt), and this space needs to be ventilated. Again,
a vapour barrier is needed on the warm side.

Correct, except that the Building Regs do not require a vapour barrier in either
form of roof - they are only concerned with ventilation.

In the Building regulations part F2, section 0.2 it states that roofs
where the moisture from the building cannot permeate the insulation, no
ventilation is needed.

So why is it not possible to put insulation between the rafters, and
then seal the warm side with a vapour barrier, and not have any
ventilation? The reason I ask this question is that we have a flat roof
running up against a pitch roof, and it is really awkward to ventilate
everything. Unfortunately it is a bit late in the day to use a warm roof
construction.

Any explanations greatly appreciated,

Paragraph 2.7 of Part F2 says "Vapour checks can reduce the amount of moisture
reaching a void but they cannot be relied on as an alternative to ventilation.
A complete barrier to moisture is needed for this". It is virtually impossible
to create a "complete barrier" in this situation - there will always be
perforations for pipes and cables, as well as the loft hatch. What you call a
"vapour barrier" is really only a vapour check.

The Regs only apply to the construction of new roofs. There's no legal
obligation on you to comply with the Regs concerning your existing roof, so
there's nothing to stop you providing a vapour barrier and omitting the
ventilation if you wish. There are millions of older pre-Regs houses without
either ventilation or vapour barriers, all running the risk of rotten roof
timbers. But if you do wish to comply, there are other acceptable ways of
ventilating besides the eaves - e.g. vent tiles and ridges, grilles in gable
ends, ventilated fascias or through-roof ventilators for the flat roof. More
info is needed on the age, design, construction and coverings of your roof to
help much further.

Peter