new plaster

what ever you do, do not use PVA, you will end up with tiles on the floor, the best thing is to fix a purpose made waterproof tile board to the wall first and tile over that.

 

Have a search through the forum as this is a very common question and loads of advice

The use of PVA in this situation is not the right choice, as phil says (and countless others)
Tile board is a good, long lasting solution, any leaks over the years and the plaster wont simply disintegrate with the damp

If you do decide to tile direct onto the plaster, use the correct sealant, an example below for you;

http://www.toppstiles.co.uk/tprod3072/section885/Bal-Primer-APD-25LTR.html

 

But doesn't he say that the walls have already been plastered so he's wanting to apply something direct to the plaster before tiling.

That primer is okay for the rest of the tiled area in the bathroom but this is cheaper, obviously smaller but better value.

http://www.screwfix.com/p/mapei-tile-adhesive-primer-light-blue-1kg/87305

But neither of these primers are going to waterproof the shower wall.

 

As said, prime with BAL Prime or the Mapei stuff, then tank it, with either BAL WP1 or Mapei stuff. I use the Mapei now as it comes with better tape. P.S. the primer comes in the tanking kit.