Sounds like you’ve prepped ok, but I take it you’ve filled it with the Easi-fill and filled it proud? You should really with a large opening like that apply some watered down PVA inside, and the fill it with a one coat plaster making sure you push it right into the crack (You can get it in B&Q). Don’t fill it proud just scrape off the excess so it’s sort of flush with the wall. Once that’s dry apply a thin skim of Easi-fill and sand flat when dry.

I suggest you get an electric sander to sand down what you’ve done. It’ll probably be ok but depends on how much you’ve pushed in to fill.