Vaxhall Astra 1.7 CDTI not staring all the time?

Over the last 5 days my Astra 1.7 CDTI has had starting problems, I can switch it on fine from cold but stop a few times then turn the key and it will not kick in, all the lights work it just chugs and will not start.
I can leave it 2 mins and try it and it will start, but there again i can try it and it will not.
Does anyone have any ideas?
Thanks