Can you describe your roofing system and cost. It seems just what I'm
after - I was considering using the warm roof insulation battens
available from Screwfix.

Firstly, it requires the entire roof covering, battens and felt (if present)
to be removed. It isn't a simple retrofit job. Basically, it can't be done
with the conventional roofing felt in place. The breathable sarking membrane
is ESSENTIAL to allow the wooden rafters to remain dry.

--------+------------+----------- Slates/tiles
--------+------------+-----------
################################# Battening
## Counter battening
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Breathable sarking membrane
----+----+-----------------------
| |
|R'tr| Kingspan Thermapitch 75mm Insulation + Rafters
| | TP10 zero ODP
----+----+-----------------------
Kingspan Thermawall TW56 zero ODP 25mm Insulation
---------------------------------
--------------------------------- 12.5mm Plasterboard

See http://www.insulation.kingspan.com/newdiv/pdf/tp10.pdf

Unventilated Insulation Between & under rafters
Figure 1a

The battens can be replaced by 18mm sarking board (with a slight reordering
of components), although in my case, that would sink the roofline below
neighbouring properties. Good for newbuild or detached properties, though.

Also, if the rafters are 100mm, you can just get away with building
regulations for loft conversions (but not new build) filling only between
rafters, although 25mm underneath is better to avoid cold bridging and
condensation, as well as improved energy efficiency.

Christian.