how can i find out wot is causing my vauxhall 1.7cdti astra van?

my 2002 astra van will turn over and over with out firing up unless i give it a squirt of easystart then its away no problem it drives spot on untill i turn it of again then its back to the easystart even if i just turn it of for a second she wont fire up my glow plug light does not come on but it never has done in the 3 years iv had it please some one help me its going thru tins of easystat like theres no to moro. chris