After much struggling to get my cornice corner work to look anything decent I followed the advice of fred and made my own Mitre Box.

I searched and searched everywhere here but nobody knew what on earth I was on about.

Made my own very simple one and what an easy job it became.

I actually followed some youtube help vids and created the corners first then measured between the faces to install the long lengths. Perfect fit.
Just a little bit of filler to smooth out the joints and it looks good.

Onwards and upwards.