have you hacked off any ares just to see if the blockwork has cracked behind it either that or try using a feeler gauge to see how deep it goes, i think most new blockwork cracks as they build it up to quick and too many courses per day the buildings haven't even started to settle before the rendering starts, we had a similar thing on a job in braintree a few years back all the plots cracked in identical places we had Weber's out and everything and they just put it down to settlement and offered a solution to repair the cracks, when we hacked the render off the crack was right through the blocks, but when they started building the second phase the brickies were using a different brand of cement (as well as actually putting in the reinforcement in every third course) and the cracking stopped