Hi, I’m very new to this and I am trying to use blender to create a model for 3d printing. My model looks generally fine but has a few rough edges below the plane z=0 which I would like to remove. I tried creating a cube where the top was z=0 and then applying the boolean difference modifier. It works mostly but creates a few unwanted artefacts. Am I going about this completely the wrong way ? Any advice would be very welcomed. Thank you.
This may work in your situation, using the transform panel in the 3d view properties shelf (N-key), in edit mode set the Z median to zero after selecting the vertices you want to move up.
That looks perfect, I will try it tonight. Thank you very much. 
Also, if vertices are not on the same plane (all have equal Z value/location) flatten those by scaling to 0 on Z axis.
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The interesting question is how to select all vertices with Z<0? Box select may not be precise enough if you have many very close to the axis. Python?
import bpy
bpy.ops.object.mode_set(mode = 'OBJECT')
obj = bpy.context.active_object
max = 0
for v in obj.data.vertices:
if v.co[2] < max :
v.select = True
bpy.ops.object.mode_set(mode = 'EDIT')