OK thanks for all your input guy's. I've factored 10 x 25 L 15% SH into the cost and plan to buy and take 6 to the job with four in reserve if needed. The block in the photo is just an annex to the main larger block next door. They are having scaffolding going up later in the summer for other works and are interested in making use of the scaffold to have the render cleaned on the remaining block at this time so I will use any unused hypo at that time.
I'll let you know how this goes and post up some finished pictures of the block shown in the photo.