Billeh Gahr's trap is not working properly in the current version.
In the room past the source abomination
there is a painting on the wall that you can not move; attack it to reveal the button.
Thanks again Raze. It seems that I looked in the right spot first time through but didn't work quite hard enough to get the result! All done now though.
Very disappointed that I didn't actually get to see the effigy bearing your name though. ...
Now that I've finished clearing out the various bits of the cave, I'm back in the same old boat though - I can get back to town but the game crashes if I try to save anywhere.
Ah well, I think I've pretty much explored most of the corners , fought nearly all the fights, and tied up quite a few of the loose ends now. So it's time to call it quits for a month or two and come back with a fresh eye when the game has moved on a bit.
Thanks to Larian for the chance to try a game at Alpha stage, and to all the posters here for posting their experiences, opinions and help. Interesting stuff.