Hi @Scout1,
A warm welcome to the Bunnings Workshop community and thank you for your question.
It's certainly not a great install and if you can have the installer come back and rectify it, that would be my first suggestion.
The issue is that the walls are either not perfectly square or perfectly plumb at the corner. You can certainly fill the gap with No More Gaps and then paint over it, but this is often quite visible and there is a fairly easy way to lessen the gap before doing this.
You can see that the gap is almost non-existent at the bottom of the mitre. If you place a chisel behind the skirting on the right and pull the top of the skirting board over slightly then use these Jack Pack'n'Snap shims to pack the skirting board out, you can then pin the top of the skirting board with a brad nail and snap off the shims. This will close the gap at the corner and hold the skirting board that little bit out from the wall.
It will open up a small gap on top of the skirting board, but once a bead of No More Gaps has been run along the top of the skirting board and painted, it is much less visible than a gap in the corner.
Let me know if you have any further questions.
Jacob
