Repairing Cracks In Wall

I would seek a professional opinion as it sounds very much like some form of subsidence due to type of crack you are describing and the fact you believe it to be a party wall. Removal of the top coatings (plaster and render float coat) will not necessarily disturb the structure, if completed by a competent person as to investigate it properly. A structural engineer will be able to identify it correctly threw measuring and visual inspection.

Speak to structural engineer. But in the mean time drill a hole either side of the crack approximately 5mm away either side -raw plug and screw in each hole leaving head of screw proud - then tie some cotton between the screws tight . Walk away and leave it for a while ( few days to a couple of weeks .. if the cotton snaps or moves from position you left it in you have movement . Personally my house does the same year in year out in the same place normally when there’s a change in the weather hot to cold or vice versa . Don’t worry too much your find it happens in a lot of properties . I do a lot of this kind of work for structural engineers . Hope this helps . JB

I'd clean the crack and paintable caulc it in if the house moves, but it won't show the crack

If there are no cracks in the brick wall then there may not be an issue with subsidence and movement of the party wall. Some houses, especially circa 1920’s are plastered with a plaster that goes brittle and powdery over time and loses adhesion to brick. There are a couple of options...remove all loose plaster and re-skim or board re-skim with new plaster. Or, rake out and fill cracks to a smooth finish and apply a woven fibre lining paper that will bridge and hold all cracks in place. We use 1m wide Wallrock Fibreliner fixed with their special adhesive. Any visible joints (should be minimal if applied properly are filled with poly cell fine surface filler and then over painted with emulsion