I fear that this may have been covered before, but can a player running the Siege of Winterfell agenda ( "you cannot claim power for your house except during a M challenge" ) still take power from an opponent's house card for winning a power challenge?

Our group is split - some say you can because it's not technically "claim" (it's a move/take), while others reckon it's still a claim.

Which is it?

The first part of your group is correct. You can MOVE as many powers as you want and that is what you do during POW challenges. Claiming power means that you bring more power to the game. But moving power during POW challenges does not change the number of powers on the table...

Thanks Rozy, we needed that clearing up. I think the core rules using the phrase "claim value" caused a bit of confusion.

Ironically, it was our resident Stark player who was arguing against being able to gain power from power challenges like this. I'm sure he'll be glad to be wrong in this case .