No but we have celotext (sp?) - a foil backed insulation board.
Just remember that the timbers need to have air flowing over them, so you need at least a two inch gap between the insulation and the felt under the tiles / slates.
If you have had air permeable felt installed this gap can be reduced.
Extratherm is pretty much the same stuff as Celotex or Kingspan - a polyisocyanurate foam board. Pain in the backside to cut - be prepared to make plenty of dust! As said previously, if you have felt on the roof, you need to provide a cross flow of ventilation at least 50mm from felt to the insulation.