Stealing a certain item from the druids didn't help me. I still couldn't talk to Mol about my stuff.
In the end I decided to just move on, and the quest completed once I'd sorted the situation at the grove, albeit with the comment that I'd not managed to get my stuff back. So at least it's not hanging around in my journal, albeit I'd have preferred to have completed it successfully.
It is starting to sound like a bug, so I'll report it and fingers crossed we'll get a fix for future playthroughs.
