Since I also have a 2.8JTD, I have taken the time to peer underneath would you believe 3 times, in order to examinethe the dipstick fitting as the dipstick entry is hidden by the blowby connection, which branches from the dipstick entry fitting at the crankcase.
Unfortunately the method of attachment is not clear. As there should be no great pressure in the crankcase, I am tempted to speculate that it could be a light push fit with "O" ring{s). It is quite likely that the "O" ring has hardened and perhaps worn with vibration.
I note that on the picture posted by DevonT2001, the adjacent blanking plug seems to have a fibrous jointing material in addition to the copper washer.
Copper washers can be softened and reused, if heated until red hot, and then quenched in water. I have used this trick many times, on sump plug and injector washers, as well as when bending domestic water pipes.
I am attaching the relevant page of the Iveco Daily 2.8JTD manual, which shows the fitting in diagramatic form