Many thanks to all of you.

The general impression I get from your replies is that, to combat heat conduction through windows, one really needs triple glazing but that good quality, interlined floor-length curtains can be effective, especially if they're sealed round the edges. On the other hand, I'm sure triple glazing won't do much  to combat  heat gains & losses by radiation, whereas curtains can stop a lot of radiation, especially if they're lined with a light coloured material.

da_getz' suggestion that pleats will hinder air currents is interesting, but I'd like to see some evidence to back this up.