Coving crooked old walls!

Hi all,
I've managed to get about half way around my room but now I'm only left with crooked walls - what do I do??
I have thought about nailing the coving in place but one of my walls is like dust behind the plaster.
Has anybody got any tips for me?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.