Badger,
What depth and dimensions will the poured slab be? This may make a difference to what strength of EPS you require (hopefully one of the SE's will be along shortly, and they'll want to know anyway)
For reference, we poured a 250mm deep, 137m2 foundation slab with UFH pipes tied to the mesh. Our insulation was the Jackodur Atlas system (240 or 260mm thick), compressive strength of 130kPa if I recall correctly.