Hardwall may be a better bet if the block work is sound; make sure you chip the edge of the original plaster so it gives a jagged edge to give a better key & don’t try & lay on 20mm in one hit as it will craze like hell & not stick properly. Build it up in 3 layers, each one applied as the first has gone off but not dried out. Use at least 2 coats of PVA on the bare block work, the second immediately before you plaster. Plaster it doesn’t have a ‘mix strength’ just a correct consistency, mix it too thin & it will end up all over the floor; mix it too thick it will go off too quick & not bond properly!
If the wall is not too big, I always prefer to fill the repair flush with the surrounding plaster, lay on some reinforcing tape all around the join (cracks tend to show again where the 2 meet) & then skim the whole wall; this will give you a perfect, level finish & no one will even know it’s been done. If not, leave the repaired area 2-3mm below the adjacent finish & skim over pulling the finishing plaster out beyond & over the original plaster to blend in (tricky); you may still be able to see the join though & there is the likelihood it will crack around the join in time.
