How do I patch a large hole in plaster and lath?
What I have done on my 1909 house (I am in the U.S.) in the same exact situation is clear a wider area of the lath and plaster and replace it with drywall.  To do that, you will need to add some padding over the studs because the lath&amp;plaster is thicker than 1/2" -- what I did was pad it with 3/8" plywood, then drywall over plywood to achieve approximately an even joint between the new drywall and the old plaster.  Then you tape and sand the joints like you would with regular drywall, prime and paint it later.