this is movement on the stair string can you put extra fixings into the wall then fill and paint over fitting a moulding will cover the first crack but when you caulk the joint between moulding and wall same problem fix at about 400-600 centres with large plugs and screws if you can access under sta...
if you have a radiator in the room hide the joint behind when you cut the skirting clamp two peices together cut the joint in one cut then cut the other end to suit
if you can dill a large enough hole down into the existing post and into you new post use a part of a brush handle leve some clearance as your hole will oy be dead straight use a gap filling glue and if possible 4 diagonal screws down into existing post
when you cut your joint hold or clamp the two peices together cut the joint then cut to length of room so your joint is behind a pipe if you cut both togther its forced to fit
