Sedan ownership - final destination of all erudite 105 fanciers !
You've probably tried this, but if there's significant tension on the steering wheel against the lock detent the key will be unable to turn, you have to turn the steering wheel enough to take the tension off the lock detent and then the barrel will be able to turn (and pull the detent back out of the way)
Otherwise I'd try a little graphite powder and running the key in and out to jiggle the pins free. Progress to WD40 if no luck
To remove the ignition barrel you have to undo and remove the cowling, then you will see a dome headed pin on the top of the barrel holding it in. It's actually a machine screw and you'll need to find a way to grip it or tap it around with a small chisel.
It's definitely the right key ?
Good luck !