allow the wall time to dry out before adding more water to an already saturated brickwork. i would also check to make sure the leak has been fixed and it wasn't a combination of leaks accruing at the same point although the brickwork may seem dry to the touch it may still be wet inside, every time you add more moisture that brings the hidden dampness to the fore, depending on the type of property 6-12 months to dry out fully

It depends on the stain blocker you used the best is a shallac based paint like BIN by Zinnzer water stains can be tricky as the water passes through it picks up any pigment of the material it goes through including rust most other stain blockers out there are not that good a decent oil based white undercoat is usually good too and make sure the area of stain is dry first and any leaks are fixed

Here's a tip from having the same problem. I had a leaky roof upstairs on the gable end and rising damp downstairs making the emulsion crack and peel.I got the roof fixed and damp coarsing done round all the 3 walls. Then
I scraped and sanded the areas finding damp plaster underneath. So I dried the areas steadily with a hair drier, when dried I primed with an oil based undercoat and then emulsion and it worked.

If completely dry use jonhstone stainaway paint ceiling all over not just the stain

The previous was ok but it could be that the loft insulation still contains moisture. If not then a slight rub down 120 grade of the affected area and apply Zinsser B.I.N. You will need a cheap clean brush. Make sure the BIN is thoroughly stirred and apply one coat allow at least 4hours and then a second coat. Cover at least 6” over the affected area.
This should definitely do it.
Thanks