There has clearly been some improvement in this area:

We went from having no clean sheets in forever and looking vulnerable to every attack, to now having 3 in the last 5 games (admittedly 2 of those were against Barnsley and Peterborough), almost a 4th against Forest who grabbed a late consolation goal and managed to keep out Liverpool for an hour or so.

I wonder whether the improvement is more to do with the midfield rather than the defence itself.

All season we've wondered if we have the players who can play in a back 3, and for most of that time the defence has been pretty poor.
There have been plenty of individual errors, but none that seem specifically to do with the formation.

When we had Flint and Nelson and Pack passing it around at the back in little triangles it was horrible to watch and you always felt that a clanger was about to happen - however I can't really recall any of the goals we've conceded were down to not having the players to play out from the back - if anything it probably hurt us more as an attacking force, as the transitions to attack were so painfully slow that the oppositions always had the change to reset their shape.

By contrast, a lot of the goals I can remember seem to have been due to a lack of protection in front of the defence, with attackers able to run at the heart of the defence quite often. This seems to happen much less often now.

We have changed the defensive line by putting Ng in there, which has helped us become more mobile and able to pass out better, but at the same time we've overhauled the midfield, and specifically brought Wintle in for Pack.

I think the latter has probably had the bigger effect.