There are two types of polystyrene insulation one is the shape of a revolton which fits between the vegas but usually only used on new constructions, the spacing needs to be right,

If its an old roof vega spacing can vary !! then sheets of polystyrene are available at Mayors the builders merchants it comes in 2m x 1m x 50mm sheets and in two grades rigid and semi flexible.

Its called Porexpan and 10 sheets will set you back about 42 € it is easy to cut with a stanley knife or a saw with a thin blade, makes a lot of mess though!!

If you really want good insulation double up to 100mm but its costly!! worth it in the long run though if you want to stay cool man!! :-[ :-[

ps the battens out here are metal instead of wood and they are fixed to the vegas with clips, threaded bar and rawlbolts the metal studding is called Maestra Knauf and comes in 3m lengths 45mm x 18mm

ps make sure you bring plenty of masonry drill bits as the vegas are a pig to drill into very tough !! :-[
and if you can,  try to find a peice of vega outside the casa wall with its end exposed, to let you know where the reinforcing rods are!! you can't drill through them!!

Hope this helps stuart

Regards Stuart