1999 Vauxhall Vectra diesel 2l Vectra wont start

Engine Mechanical problem
1999 Vauxhall Vectra Manual

my vectra wont start, it turns over like its going to start but just wont. People say its the fuel tank which I really hope not as they cost a small fortune.
there has been no lead up to it not starting it always starts first time and has ran perfect up untill a week ago when it just wouldnt start. Mechanic has had a look and says that the diesel is gettin to all the correct places it should, reason for thinkin it is fuel pump someone done the diagnostic testin on it and that was what it said, how accurate are these?Should we get another diagnostic test done just to be sure?Would be gratefull for some advice.