John
There will be others along who have more knowledge than I , but I have been experiencing this problem intermittently for a year or so.
Each time I am just about able to get the key to turn by "waggling" the steering wheel from one side to other at the very limited amount of travel permitted.
I reported this to a dealer and they said that it was a fairly easy job to replace the barrel. There is no interlock with the ecu on my +8 , I suspect this would be similar on your Roadster.
Sorry , no help on part numbers.
