Got a 1997 Vectra Estate 2l diesel

Had some trouble some time ago and changed the crank sensor and spill lines and the thing started up and ran again

Since that time I noticed that, from cold it took a lot more turning over to get it going - when it was hot it was back to its old self and started first pull

Had a problem when it woulnt start a month or so ago, but left it overnight, charged the battery and it started after a while

Now things have got serious (recovery vehicle serious) I have been all day trying everything to start the thing opened one of the injector lines and not much was coming out - in case there was a blockage changed the fuel filter (filled it up with fuel to reduce the bleed time) turned it over, it tried to start - so I bunged a bit of ether in and after turing over it ran!

so joy of joys got changed to go out and ---- you have guessed it when I Came out again and it would not start - Thought it was air so nearly flatted the battery but no joy

Got changed into my overalls again - no leaks as I could detect but when I took the input line to the fuel filter out nothing came out as I turned the engine over.

Now my question is where does that line go and what pumps the fuel through? could it be blocked? Does it come from the fuel tank? If I pour fuel in will it prime the pump?

Any ideas appreciated.
Thanks