first Q, what happened to the old cable?
is there a galvanised backbox in the wall? if so, take it out.
Although you can fill the entire hole with plaster, I prefer to mix up some sand and cement in an old butter tub or something. Clean out all loose material and dust from the hole and spray it with water to dampen, then press the S&C very hard against the back and sides (if it is too wet or too dry it will fall out). Once it has stiffened you can apply more, starting at the bottom. It may slump a bit, so leave a gap at the top big enough for you to press more in once it has set. you can use a filling knife. If it sags or shrinks away from the wall, press it back hard or it will fall out. S&C stays workable for hours, you may need to dampen it a little, so is easier to work with than plaster or filler. If you get it wrong you can scrape it out, wet it, and start again. It does not stick and drag on the knife. Set the level less than half an inch below the level of the plaster at the sides so you will have no need to scrape or cut it back. Leave it a day or two to harden. You can then apply plaster (buy the smallest possible bag, it will not keep for long)
Thick layers will shrink or crack. You can overcome this by putting a layer on that's just a fraction below the surrounding wall, then, when it's set, apply a thin, quite runny coat to fill any cracks and level it off. Any excess water will be absorbed. I use a plasterer's trowel when filling chases and patches, it is big enough to span across from the flat wall on either side so you can press or scrape any excess smooth. When it looks set, flick water on it with a brush or garden sprayer and polish the surface with your trowel. A broad filling knife can be used if you don't have a trowel. Plaster must be used at once or thrown away. Don't try to wet and reuse it.
Do not leave any plaster on the wall or patch that projects beyond the surrounding wall. You will have to scrape or sand it off so it will be wasted effort and material. If the surface is a bit low you can easily add a bit more later.
btw to remove any excess, shave it off with a broad metal knife while it is still cheesy. This is less effort and makes no dust. Hold the knife pretty flat so it works like a plane or razor.
