Do you have a feeling as to whether this is because the bottom
modulating point is less or because it's condensing more internally
and delivering to the condensate drain?

I'm not quite sure. We were going to pull the condensate drain apart and
take a peek, but never got round to it, as I needed the cooker point moved
the same day.

One major difference is that the flue is much longer. Not very long, mind.
However, it does have a couple of bends, an extension piece and a vertical
flue terminal. This would give a much greater chance of flue condensing.

I still don't know if it isn't pluming because the flue gas is too hot, or
too cold for major condensing at the terminal. If anything, there are more
wisps at higher return temperatures than lower ones.

It seems a shame really, having gone to all the trouble of having the
terminal in a place where pluming wouldn't be a problem! (It goes out
through the roof).

Christian.