Radio programmes downloading fine with relevant thumbnails, but TV programmes are no longer adding the thumbnails. Has this feature been removed or do I need to specify something now. Or is it broken?

No, it's not broken. How did you determine that there are no thumbnails?

I think I can see what's happening now.

I download this, for example...
http://www.bbc.co.uk/iplayer/episode/p04...-6-memoirs

But I don't get the image shown at the above link for the specific episode, which I used to get.
I now get a generic 'series' thumbnail which seems to come from this page...

http://www.bbc.co.uk/programmes/b07h1mnp

That is by design. Brand/series images are now used instead of episode frame grabs. I neglected to add this change to the 2.95 release notes - now corrected at GitHub.

Okay. Is there any way to force the program to download the episode thumbnail rather than the series one?

No. However, if you want the image for a particular show you can find it in the HTML source of its iPlayer episode page (e.g., http://www.bbc.co.uk/iplayer/episode/p04...-6-memoirs). Search for "og:image", which will locate this:

<meta property="og:image" content="http://ichef.bbci.co.uk/images/ic/1200x675/p042112x.jpg"/>

That's cool. Thanks for your help.

Is there a way to update the thumbnail manually?

I also notice this causes problems with TV programmes that have no series thumbnail, such as http://www.bbc.co.uk/programmes/b07mnbys...des/player. Get_iplayer finds no thumbnail so downloads none.

That is a BBC problem. They put a series image in the programme metadata and then misconfigured it, so the image download failed. You can tell something is wrong by looking at the series home page. There is a blank space where the series image should be. It happens sometimes, particular on the kiddie channels. You can fix it up manually, of course.

This happens e.g. on downloads of the Mary Whitehouse Experience (series PID p028s846). When you request https://ichef.bbci.co.uk/images/ic/480x270/.jpg what is actually served is a PNG image.

In this case, where get_iplayer warns that the thumbnail downloading failed, is it possible to fallback to the individual programme image?

Maybe, but two broken images don't make a compelling case. Besides, there is no reason to think they won't break the occasional episode image as well.