Thread: Opening not quite level

I don't know how I let this one slip through unnoticed, but, the top of the opening I have created for our new walk in pantry is not quite level.

I spent ages fart-arsing around with the framing to make sure it was just right, carefully cut and put on the plasterboard and external angles, applied the first base coat to all the edges and corners.

The I stood back to admire my handy work and thought to myself "that looks a little off", got my level and discovered that it's 5mm or so out across the 650mm opening.

So, my question is this. Is there a non-destructive way of sorting this out? I really don't want to have to pull off the angles and plaster and start again.