External insulation/render advice

Hi I have concrete block outbuilding that has been externally insulated and rendered when the house got done before we bought it. I'm going to be changing the roof on it and making it slightly higher so there will be Abit of patchwork to do on about 1 course of blocks. It looks like a grey foam/polystyrene board about 50mm thick. My questions are

Is this a certain type of insulation board can I pick this up from my local wickes or b&q (expanded polystyrene) ?
What render do I need to go over the insulation ?
I've never rendered before but with it been a small job I'm going to give it ago myself.

 

Sounds like graphite EPS... I'm sure loads of thicknesses are available but I've seen 40mm, 90mm & 100mm about.

When you patch the render, it will always be visible, could you make a very deep fascia or something so no further rendering is needed?

 

Hi yes it will be getting covered with a fascia but with me adding a course of blocks higher to change the roof to slope from back to front instead of left to right the blocks will show above the rendered walls and will be set back further than the current render if that makes sense without the 50mm insulation boards and render