Hi @Lambie1971,
Unfortunately, it would be impossible to know the extent to which you would need to go in order to repair these cracks without removing the render and seeing what lies beneath. Cracks like this on a rendered wall indicate movement of some kind, but it is impossible to really know how much movement you are dealing with without removing it and looking at the bricks.
In saying this, the hairline cracks shown in image 1,3 and 4 don't look too bad, and they are likely just due to slight movements from the impact of the door or window opening and closing. This impact will cause the brick to move slightly in relation to the render.
In the case of the hairline cracks, I'd suggest you simply v groove them with a utility knife, ensuring you wear gloves and long protective clothing to reduce the risk of lacerations, then apply some mesh tape and re render.
The crack in image 2, however, looks a bit more significant, and it would likely require you to remove the render and inspect the underlying brick to see whether a more structural repair needs to be carried out.
If, once the render is removed, the bricks themselves appear sound with no stepped cracking through the mortar joints, you may not need anything as invasive as reo bar. In those cases, repointing any loose or cracked mortar and re-rendering with a quality repair render such as this Dunlop Premix Render Joint And Patch would usually be sufficient. Using a fibreglass reinforcing tape embedded into the new render over the repaired area can also help reduce the chance of the crack reappearing.
If you find there is a more significant structural issue once the render is removed, then it would need to be addressed based on what is happening. Without knowing, it is probably best to have it checked out by a builder who is experienced with both brick construction and rendering.
Allow me to tag @Nailbag and @Dave-1 to see what they think.
Jacob
