Shift in building structure

Hello

I have a garage at the bottom of my garden. There has been a shift in the structure over the years and a builder has been to look at it. There is a crack in the block work and the builder suggested tacking either side of the crack and tieing thread tightly to the tacks. The sooner the thread snaps, the more serious the issue is. Obviously, I need to use a thread with no give. Would cotton do the job or is there something specific?

The long term solution suggested by the builder is inserting rods in the block work to prevent any further shifting but he wants to gauge the severity of the issue before making a final decision.