Preparation:
First of all go out and get yourself:
1 bag of readymixed mortar
A small bottle of plasticiser
about 4 lightweight building blocks
a small bag of finish plaster (make sure it's suitable for backgrounds of high suction, something like "Hardwall" would be ideal)
Next, if it's not going to cause too much inconvenience, I'd remove that cistern until the job is done.
Hack off the surface plaster around the hole to form a border (about 2" wide). Don't worry about making this neat. If it has an irregular shaped edge, it is actually easier to blend in with the new plaster.
Using a saw, Hammer, bolster, chissel, or whatever takes your fancy, Cut a piece of building block to fit into the gap over the soil pipe. (allow for about 10-15mm of mortar all around it, but you don't have to be too acurate). This wants to have a flat top to it, (that will be level with the flat top, to it's right).
Soak the wall all around the hole, Mix up some mortar (adding a dash of plasticiser to the water before mixing).
Butter up the bit of block you have just cut, and put it into position. Note, the front face of this should be flush with the front of the blockwork (ie, about 15mm behind the face of the finished plaster), NOT the front of the wall.
Cut up and lay some more bits of block to fill in beside this first piece, and form a nice level top face.
Cut and lay some more (larger pieces this time) to lay on top, and fill the hole completely.
Rake out the joints so that they are set back about half an inch from the face of the blockwork, and leave overnight.
Wet the wall thoroughly and mix up some more mortar.
Spread the mortar on the wall and get it as flat as you can but about 3mm shy of the final face of the wall.
Make a 2 sets of score marks in the render using the corner of your trowel. One set at 45 degrees to the right and another at 45 degrees to the left. This should give you a nice diamond pattern to act as a key for the plaster.
Leave overnight.
Wet the wall again and mix up some plaster. Spread the plaster on the wall right across and beyond the edge of the hole. Scrape off the excess with a straight edge.
Roughly smooth over and leave untill it feels quite firm to the touch.
Smooth over again, give a light spray of water and polish off with your trowel.
Leave about a week and paint the whole wall.
