Warning! Confusing and circular descriptions ahead.
What we (in N. America) call a jointer is used to make one face of rough and often twisted, cupped or bowed materials flat (or planar) and often to make one face and one edge of a board square to one another. If you run one face of a board through a jointer (aka JOINT THE BOARD) and then flip it over and do the same to the opposite face, there is absolutely no guarantee of the faces being co-planar when you're done, and the chances of such happening are probably very small.
What we call a planer in N. America, doesn't make things planar, it just makes things thinner (though one can employ various jigs to use a planer to make things planar). So, if you put a twisted, warped or cupped board into a planer, a thinner twisted, warped or cupped board comes out the other end. However when you put the perfectly flat face that you created with a jointer on the reference surface of the planer (it's bed) a thinner, board with two flat and co-planar faces comes out the other end.
I suspect Eoin may be outside the US which would confuse matters further.
In much of the rest of the world they call a jointer a planer and they call a planer a thicknesser - so the machines' names actually described their function more accurately.
http://www.recordpower.co.uk/categor...--thicknessing
Eoin, you probably want one of each if you're doing work that requires square and co-planar solid wood stock (if you're making Windsor chairs or small boxes or casework from sheet goods or whatever, it might not be needed). If you can only get one, get a planer and find another way to joint boards or take a straight edge and winding sticks to the lumber yard to pick out the flattest possible stock (there there is no guarantee it will stay flat since wood moves). Of course the other option is a combination machine which incorporates the functional of both into a single machine (often at the same cost as two individual machine but with a smaller footprint).
good luck
-kg