I am making ground for a level, and it's built up of several brushes that I have altered the height of the corns of to create slopes etc... however, when I compile and run the map the ground looks different from how I have it in hammer..

What's happening?

It depends on what you mean by "different". However the cause is probably the same... the angles of the brushes. Certain angles cause visual errors in the game.

The solution is to build your terrain differently. You should be using patch meshes and not multiple brushes. Those brushes will kill your frame rate.

Make a large brush and open the texture editor. Take a look at the advanced features of this function. You'll see you can subdivide a brush and then manipulate this single brush to accomplish what you are doing with multiple.