I have acquired some polystyrene (don't know manufacturer) I want to use it to insulate an external single skin wall and wish to render with a bagged product such as K-Rend. Has anyone got similar experience of achieving this?

how thick is the insulation? i once clad a building with 90mm (i think) celotex and then fixed battens on top of the celotex and using 125mm screws fixed into the solid wall behind. i then clad the battens with breather paper and eml before rendering. it worked, and is still working but i would try and find a fixing that is more suitable for the job than just long screws and wall plugs that i used.
i am guessing you have already dismissed insulating on the inside?

You will find plenty of pdf's on the K-Rend website

Plasterer London

You will find plenty of pdf's on the K-Rend website

Plasterer London

Carpenter London