I dig out if masonry then masking up either side and shoot with chemical fixing. When that's set toupret powder filler with mesh tape over, sand and paint. Cheap enough if small tub is enough although sometimes I've needed 2 or buy the bigger one, but that needs the specific mastic gun which you won't have so stick with the standard size
Picture would be useful. Always a good idea to try and find the cause of the crack...just in case there is an issue elsewhere. Whereabouts in the house is the crack?
it's a wall on which a chimney breast was removed, it's a terraced house. They don't have a crack next door. I am afraid to look too much into it in case it's a horror story so basically want to hide it, fill it as best I can and hope I'm out of here before it reappears!
Be brave, remove the plaster to see extent of crack. Look up helibar/crack stitching. It’s very easy to do and quite cheap. Maybe just what you need to make a permanent repair that doesn’t keep cracking.