In reply to Hodgey:
Whether you buy or hire it's worthwhile having a good think about what you do and what sort of functionality you might need.
If you just want a call the cavalry device then a PLB is the chap. It can live in the bottom of your sack and there are no additional charges.
If you want additional functionality like tracking or sending preset messages via satellite then a spot tracker is more useful.
I'll caveat my advice by saying its a year or two since I did my research.