mmmm, big Laminge outside doing a bit of labouring eh!!. There is a range of products that you can use which are normally used on Hebel block and sheet products. To get the effect you're after (i think) use a scratch coat product. Its pretty much render already made up that has a certain size particle in it, ie 2mm. As you trowell it up the wall the particles set into the render and form an effect like someone has scratched the render (with 2mm fingernails!!) Contact CSR, they make Hebel and all its associated coatings.
Mess with the bull, get the horns!