Hi
I only have data for the earlier X250 models, but the procedure will probably be similar for your X290. The "book" method calls for removal of:
Centre Front Grille
Front Bumper
Front wing/headlamp housing crossmember (Bonnet release catch and operating cable can usually stay attached)
Throttle body and/or EGR valve at front of engine
No mention of radiator - on the X250 this can be eased forward a bit on its hoses to give more room.
The plugs themselves sit just above the inlet manifold (see pic) and like many things on the Ducato are hard to get to without a lot of dismantling. As they are well recessed, you need clearance to get to them straight on. I guess it's the price paid for an engine bay with no "wasted" space, where the whole engine/gearbox assembly comes up from below during factory build.
On a relatively new/low mileage van the plugs should come out easily, but they are a fundamentally weak design (long and thin, with a thin wall) which can easily seize in the head after time and mileage so care is needed. If you are having this job done under warranty so much the better.
