You can try to use plaster washers to re-attach the piece, but it is likely to break and be a real PITA to finish out so nothing shows (a whole lot of feathering will be required).

Some drywall can be attached to serve as a base, then use setting type joint compound mixed about as thick as peanut butter to bring the area flush.

Setting compound will not shrink anything like pre-mixed mud if you use just enough water to max it workable.

Easysand is not hard to work with, and can be sanded.
Durabond is as hard as lime-putty plaster and does not sand well at all (plaster was not sanded to make it smooth either).

Thanks for your reply but could I embed this piece of plaster in some easysand on the wall, let it dry and then continue to fill in any cracks or spaces as well as smooth over the plaster with easysand? Will the easysand adhere to the plaster?
If so, do I have to wet it first? Thanks

"could I embed this piece of plaster in some easysand on the wall,"

Easysand will not stick to the lathe well and form keys without adding fiber.
Chopped fiberglass works sometimes, but you will also need a bonding agent (Euco Weld is one, really just thin polyvinyl acetate glue like Elmer's) to adhere Easysand to wood lathe.
If the piece is full thickness adding anything behind it will require a large amount of feathering to hide the repair.
If light washes down the wall it is very hard to hide any feathering. The 'bump' will show.