I've managed to accidentally lock the extrude function in Blender to a single plain of movement.
When I select a face and then select Extrude by pressing the E key a blue line appears and I'm only able to extrude in a straight line along that particular plain. I'm not pressing XYZ to purposefully do this and don't know how I turned this function on.
It's as if I've switched on the orthogonal mode in Autocad.