Thanks noki - hubby did the steel frame as we had some leftovers and got some extra steel free on Gumtree. I think that wood is good as well - you will need to make sure it is treated. Just a matter of what you have available.
sorry missed the second part - we want a nice finish to the feature and even though we are using blueboard for the wall we want it to look like solid wall so the roll on render might not cover the joints enough to make it look nice. Same with the eco bricks the render needs to be thick enough to cover the cement lines.