I have a rover 45 and have problems starting it on cold mornings, but once it has started it is no ploblem for starting through out the day it's only that first thing in the morning. I have done a couple of obvious jobs like changing the glow plugs-and relay unit, and a new fuel pump. but still no joy. The car will start but it taking quite a few turns and this in time is flattening the battery after a week or so.
Did you check the power supply to the glow plugs ?
Maybe a compression test will show up something if there is a lot of miles on the clock.
Does the car turn over fast or sluggish ?
If it is sluggish maybe the battery is past its useful life or the starter motor may need attention.