Done all checks but still have some penetrating damp to third floor flat. Researched external barriers such as storm guard and storm dry which claim to still allow the wall to breathe. Does anyone know how effective these sort of barriers are ?

We bought a new house situated about 800 ft above sea level in the Pennines. The horizontal rain we got there saturated the outer wall bricks and in places we had damp patches where water crept across the wall-ties.

The builder applied silicone-based waterproofing fluid which was effective for the 4 years we lived there (yes, taking bricks out and cleaning/altering the wall ties is the proper fix for the problem)

From reading the posts so far it seems that you have solid walls with an outer skin of stonework. On the 3rd floor you have two possible causes. Wind driven rain coiming straight at the wall will penetrate eventually. If the pointing and stone are on good condition there is little to be done apart from trying a breathable coating. However the weather has to be quite extreme for an extended period to penetrate fully into a solid wall of at least 12 inch thickness. The two most likely causes are the bottom three courses of slate, which are directly above the wall or the wind is blowing the rainwater over the guttering and into the wall. Check the slates for damage and adequate overlap. Second cause is that slate roofs generally do not have sarking or underfelt which bridges the gap between the bottom edge of the slate and the centre of the gutter. As the water spills from the slate and before it lands in the gutter the wind can blow it against tthe wall. No sarking, no protection from this problem. It is fairly easy the install a flashing, which can be inserted under the slate and finished in the guttering. Lead is best but there other materials which will do the job, including roofing felt. I would try this first. At least then you can be sure the water is going into the gutter as intended and not down the wall. If the damp still persists when the rain is wind driven against the wall, then consider the expensive coatings. Hope it helps.

Concrete