I recently redecorated a room, raked out the crack filled with polyfilla and the cracked returned within months.
The cracks never get big so it doesn't seem to be structural movement. May be it is heat expansion.
So I hit on the idea of caulking the crack instead of filling. It is mainly hidden behind the radiator so a minor imperfection doesn't notice.
Was this a sensible idea of will the emulsion paint crack on top of the caulk. I just read the caulk tube and it explicitly says not to do this :o(.