I'm using chipboard from an old kitchen unit to box in exposed pipework in my bathroom. My measuring was pretty spot on but then I found that near the floor the wall goes out of true and therefore is not quite perpendicular. I tried to plane the edge of the melamine chipboard to make it fit snug to the wall but it's as hard as hell. Admittedly, the plane I'm using is a cheap one and probably not up the job but is there an easier way to do it? Thanks.

Belt sander

The melamine itself is very hard and will quickly ruin a plane blade so, as has been advised, a belt sander is a good way of removing a small amount of material without ruining the melamine edge.