The cut-up water bottle idea turns out to have a fault - it seems to confuse the plants by exposing the roots to some light, in our case anyway. Growing in see-through pots seems to hindered the plants' development after a certain stage. We started moving the chards into shop-bought flowerpots and they are instantly showing more will to live.
The radishes remain in the see-through vessels, because root vegetable don't usually like being transplanted. It looks like their roots will never turn into radishes though - but I'm still happy because they are growing a lot of big leaves and I've been wanting to try Steph's radish greens pesto idea since I've read about it. (I'll tell you how the pesto worked out next week.)