fuel?

hi,i have just got a c5 2.2hdi(had it given)it wont start.
occasionaly it fires up and runs great,so heres the question"is it fuel pump??the fuel is at the top end filter
and seams to be in both pipes
but most of th time when you turn the key it fires up
but wont run,it did have air in pipe but it seams to have cleared
"where do i start to look"