Thanks again, LR.
I'm still unsure. One thought occurs to me, that perhaps putting on a bit of opposite rudder when established in the banked turn mobilises the available stabilising effect of the very slab-sided fuselage of the Cadet, so the rudder is virtually acting as a 'trim tab' and reducing the sideslip and height loss. This unlikely-sounding effect, if it occurs, would certainly never be able to act if the glider were an EoN Primary, which has almost non-existent side area of the fuselage!
It makes you think, doesn't it? Any other contributions to this retro look at what we all did without thinking too much about it, in those wood-and-fabric contraptions?