Thanks, chaps. I have done a bit of reading myself and have found that Aerad seem to be responsible for some of the confusion. SSA is Aerad pre-2000-speak for MSA (Means "Sector Safe Altitude") but is always based on ARP. Aerad also have two definitions of MSA, one means Min Sector Alt, as you have described - the other means Min Safe Alt in connection with their "MSA Contour Envelope".
Your input most useful to confirm what I think I understand from the books.
"Safety Altitude" seems to mean different things to different people, would I be right in thinking it's only for when you are lost?