Not a diesel but this issue is common with E300/210’s as well.
On our E320/97 the ignition switch suddenly became inoperative, it would take the key but would not turn. Anyway I struggled with the key for 15 minutes and finally got the car started, went straight to the dealer. Dealer had to get the replacement tumbler from their central parts location so the car stayed with them for two days. According to the dealer I was lucky because the key was not stuck in the ignition, things get considerably more complicated since they have to actually cut the ignition loose.
The total bill was $324.00.

Vahe
240D 77 355K