I've finally worked out a relatively easy way to mitre a corner, so am posting it here, not only in the hope it might help someone else, but so I can find it again after I've forgotten how I did it.
First mark out the size of your cloth and press up the hem. I'll refer to this as the "hem fold" in future.
Next measure the depth of your hem, and press in the raw edge. If your fabric is "springy", you'll need to baste this (stitch with large stitches to hold it - basting can easily be pulled out later), as it must remain folded.
Turn your fabric over to the right side, and put a pin at the corner. It's really important you don't lose the corner.
Now bring the hem folds each side of the corner pin together. Don't worry, you're not really going to join them, this is just to work out the angle of the mitre. Making sure they're exactly aligned, pin them on the inside of the hem fold. It should look something like this.
Now, turn along that pin line, and pin the hem allowance at the same angle. This is your sewing line.
Sew from the corner pin to the edge, over the folded or basted raw edge fold. Backstitch or lockstitch.
Finally, remove any remaining pins, trim the seam, turn inside out (which is actually right side out) and you're ready to hem with a very tidy mitred corner!