These are not original parts, but according to SJs something similar was used on later Esprit doors. They glue onto the outside of the window frame to provide an extra seal. Since my windows have a habit of whistling and leaking a little air at high speeds, it seemed like a good idea.
Until I came to dry fit them - when it was immediately clear that they wouldn't come close to sealing against the door aperture.
I could fit a finger in the gap; the seal barely protrudes from the groove it was sitting in. So totally useless.
If anyone has got a window frame that is a much tighter fit to the aperture and thinks it might work for them (or has an Esprit they think it might work in), you're welcome to them for the cost of postage.