btw if you are repairing electrician's chasesand fillig round boxes, yoiu can use Easiplast, or, easy but more expensive, Plasterer's Joint Cement which is intended for filling cracks and joints in plasterboard. You can buy it, ready mixed, in a big (heavy) plastic tub with a yellow lid. Keep the lid on.
plaster products are much cheaper, and much better, than DIY filler.
clean and scrape the wall first. Slodge your plaster into the hole and use a wide metal filling knife (a plasterer's trowel is better if you learn to use it) to press it, very very hard, into the chase, running the knife or trowel so it is flush with the existing hard plaster on both sides. you must not have your new plaster sticking out. If you can't press it flat, use a broad metal scraper, like a razor, hard against the existing flaster on both sides of the case, to cut it off, while it is still soft and cheesy.
The first layer has not got to be smooth and perfect. It may shrink and crack. You can apply a thin second layer, as soft as custard, to press into any blemishes once the backing is hard.
Even easier, mix sand and cement mortar, and use it for your initial fill, scraping it back to 1/4" or so below the finish level. Mortar is very easy to work with and does not set hard quickly, so you have plenty of time. It will take a couple of days to harden, after which you can skim it smooth with plaster.
All your tools must be kept perfectly clean and bright and you must never wash plaster or mortar down a drain or into a sink.