I've been following this Thread without comment. The article in Dr. Wankers' link by Michael Yardley has good advice. I also have Yardley's book. "Gunfitting", and one thing stands out.
I can't recall anywhere Yardley suggests shooting a bird or clay target gun off sandbags. To get a good representation of point of impact from a pattern board, it seems apparent that shots at the pattern board should be taken using the same mounting and shooting style one would normally use on birds or clay targets. Save the sandbag rest for the slug gun and rifles.
Keith, I'm not sure if you were referring to my earlier post or not, but I think I'm the only one who mentioned shooting off a sandbag. Let me emphasize that the only purpose to that is to see if the gun shoots straight, and I think a sandbag is the easiest and quickest way to do it.
I once bought a SxS that cross fired so badly that it was worthless as it was. Shoot at a can 30 yards away and you wouldn't get a single pellet in it from either barrel. The left barrel threw the entire load to the right and the right barrel threw every pellet to the left. It took 2 shots off of sandbags to determine what was happening, and I think that's a good test for any new gun, but especially a SxS.
It takes only a few minutes to carry out this test, and I think it's worth doing. If the gun shoots straight, proceed from there to see how it fits.