I have had this issue with my van (2000 Hymer A class). Whilst the Hymer is based on a Ducato, I believe the wiper system to be from Volvo (truck?), but I would imagine them to be similar in construction.
The circlip mention above is accessed by removing the nut and spring washer holding the wiper arm, and pulling the wiper from the spindle. Under the circlip is a washer, then a number of packing shims, which in combination with a wavy washer at the other end of the spindle, take up any play.
Where the spindle emerges from the sleeve bearing tube, there is a thin O ring, pressed into end of the sleeve by the shims/washer. In this system, it is the combination of the O ring and packing grease around the spindle which prevents water ingress.
In the Hymer, the drive arms to and from each crank on the end of the spindles can be popped apart at the ball joints to allow individual removal of the sleeve bearing/spindle assemblies. This way they can be overhauled on the bench.
Access to the mechanism is probably more restricted on a Ducato, but it may be possible to partly disassemble the spindle/sleeve bearing assembly to regrease the spindle and replace the o ring (if fitted), without removing the whole system.