Advice re damp interior wall/rain coming through exterior brickwork

If the correct mix designations were observed, it is a certain improbability that rain is driving 'through' the dashing. Generally a 3.5:1 water proofed scratch coat would have been applied, with a final receiver coat generally at 4:1. This two-coat mix designation is more than suitable for use in areas of a wind-driven rain spell index in excess of 75 litres per m2 per spell. (Unless you live in a lighthouse, I think you'll be okay with that level of weather resistance.) Upon closer inspection your are more likely to find plastic deformation fractures in the receiver coat and/or separation at the reveals where the render abuts the frames (caused by thermal expansion and contraction). From a distance, cracks are more difficult to spot in dashing - so I would certainly recommend a close up inspection. You're looking for physical defects in the receiver. 'Stress points' around the window are obviously highly prone to plastic deformation fracture in hard cement renders. Being a bit in the dark here without any photos to go on, I would also be tempted to look at the original rendered section (above the window) and at the demarcation detail between the two systems. If you look you will find ingress points.