Hi
The glow plugs used on the Ducato are long, small in diameter and thin walled. Once they have been in place for a few years it is very common for the torque needed to remove them to exceed their breaking strength. Result, snapped off plugs and a world of pain involving specialist drilling operations to extract the remains. On top of that, their location demands a fair bit of dismantling just to gain access. Nowhere near as straighforward as changing a set of spark plugs. I would only entrust this job to a workshop who had successfully done this exact job before. They will need skill, patience and good quality penetrating spray (there are much better formulas than WD-40).
I would want to be very certain that there were at least two failed plugs before embarking on replacement. Plugs can be tested in-situ. A very basic test is to use a test meter, the plugs should read about one ohm each when cold. However, this won't detect faults when hot. A workshop test with a dedicated glow plug tester is better, as they are run up to full temperature ( One glow plug draws 10 to 20 Amps, i.e. quite a whack of current )
From your description, unless you are dealing with very cold weather it doesn't seem to me to be a glow plug issue. When you crank a healthy diesel engine quickly enough, the compression of the air as the piston rises considerably raises its temperature. At the same time, heat is lost into the cold metal of the cylinder walls and head. If these are too cold, the peak air temperature may just not be enough to ignite the fuel when it's injected. That's where glow plugs help, by adding 100 Watts or so of extra heat directly into the chamber. Once the weather warms up, or the engine has already been running, glow plugs are not needed to start.
I think the first thing I would check is that the engine is cranking over quickly enough, it should sound quite vigorous. For this you need a healthy starter battery with a good state of charge. In addition you need good connections in the entire starter circuit, from the battery to the starter motor and back through the earth connections to the battery. The earth connection is from starter motor to engine block via bolts, then from engine to body via a flexible earth strap, and finally from body to battery negative. Check all of these, and if the engine to body strap is more than 5 years old throw it away and fit a new one, they can't be cleaned internally. It goes without saying that the starter motor must be free of faults.
If you still have problems despite a vigorous cranking speed, it's time to look at fuelling or possibly loss of cylinder compression if it's a high mileage engine.
