Starting issues Astra 2.0 Diesel

[fossway]

Hi, I am unfortunate to have bought a 2002 Astra 2.0Diesel.
I was aware that it was slow to turn over and presumed it was the battery,however after a 30mile drive it was still no easier to start. Would turn over very slowly for 10-15seconds and then fire. started like this when warm 5 or 6 times. However when cold, it needs a jump start.
had battery tested and its fine, correct type and size etc and that the alternator is fine

Have had glow plugs suggested but I would have thought they would have only been of use when engine is cold

any other suggestions? same slow turn over whether hot or cold

thanks