If you go back 2 pages, I already suggested that may be the case, but if you're still not sure you obviously didn't read my post. You can check where you are up to by looking at the quests completed. Go into the saloon, click on the Sheriff and scroll down to the completed quests to see if Kate's wedding band (the thief) is listed there. If it is, then yes, you have already defeated the thief, and you can click on it and read the epilogue to see that you now need to mine 5 bars of silver. There is no reason to be uncertain of what to do next or where you are up to in any quest sequence when the information is all there for you in the saloon.
Sorry, I had read the thread but a while ago before I tried out various things.
The confusion stems from the fact I've still got "Kate's wedding band (The unknown thief)" in Open Quests which has the same dialogue saying "Act like the thief etc" which is what I was trying to trigger.
However, I do have "Kate's wedding band (The thief)" in Completed Quests which says:
"Sheriff John Fitzburn: Very good. You are unarmed, it is after 7 pm and you have some money in your pockets. In 20 minutes I'll look after you."
And epilogue:
"Sheriff John Fitzburn: It was a woman. I was desperately looking for a man. She didn't have the ring anymore though." (no mention of silver mining next)
So I figured that was dealt with.
Only the conditions for "Kate's wedding band (Silver mining)" mentions needing silver, but not that that needs to be done first.
Maybe the conditions for "Kate's wedding band (The unknown thief)" should mention first doing the Silver mining quest as opposed to still asking you to imitate the thief. Either that or call them something totally different (The unknown thief) and (The thief) sound like the same quest TBH.
Anyways, I'm glad it's all resolved and thanks everyone for your advice.