Hannah you only have a few options.
The first is as Myla suggested, remove the render that is beneath the tiles, I asume this render protrudes beyond the plaster surface above the tile line. Then redo the render etc as suggested.
The next option is to to stick on plasterboard over the entire wall, see
http://www.how2plaster.com/stickonplaster.html . This option is only if there are no tiles. It aslo would mean replacing the cornice if any.
For the information of others reading this post, plasterboard or villaboard where tiles are to be installed have to be attached with screws or nails without adhesive.
The other option is to batten out the wall with a 16mm metal furing channel and clips, and screw fixing plasterboard. This option also means replacing the cornice.
If you patch the render and hard plaster over you may end up being able to notice the diffence in surface texture from the old plaster that has had coats of paint, to the new finished plaster. A fluffy roller to add a slight texture to the entire wall may help with this.
Some of the painting experts might advise on this.
Cheers Rod