How to monitor a crack
Number 1 Is simply a 9 inch by 4 inch piece of 4mm glass with a ¼ inch hole drilled in each corner. Your local glazing merchants can make this for you. They may also be able to provide you with rubber washers to use for protecting the glass when tightening the screws. Do not tighten the screws too tightly for obvious reasons and take care when fixing, but pinch the glass up carefully to the wall. Any further movement will cause the glass to break. You should then contact a qualified Building Surveyor or Structural Engineer. Alternatively, the glass strip can simply be fixed with a blob of Araldite at each end, provided that the wall and the glass are thoroughly cleaned.
Number 2 Is a piece of tape stuck over the crack with a pencil mark registering where each end is. If the gap opens the tape will slide across the wall and move away from the pencil lines. If this happens you should call a qualified Building Surveyor or Structural Engineer.
Number 3 Is even easier. You cut a timber wedge and tap it into the crack fairly tightly. Make sure the wood is very dry when you do so. If the crack should open any more then the timber will simply fall out. This method is the least certain as the timber will shrink and expand, as timber does, but used in conjunction with number 2, will tell you if you have movement or not. But don't take chances: if you are unsure, contact a Qualified Building Surveyor or Structural Engineer.
How to repair cracks?
Vertical Cracks – Repair using ‘stitches’ bonded into slots cut across the crack line at right angles, usually in the mortar bed joints. The so-called stitches are usually epoxy-glass or stainless steel rods and they are bonded with epoxy or polyester based injection resins. The remaining joint and the crack itself is then made good using a soft mortar mix incorporating a shrink proofing agent, to allow the inevitable small remaining movements to be accommodated.
Horizontal Cracks – The chosen repair method will depend on the cause of the crack, but the most likely cause is cavity wall tie corrosion, so replacement of the wall ties will be the primary task, not forgetting isolation of the outer ends of the originals