Hiya - if this is nothing to do with the theme apologies and I'll keep googling.
This SEO plugin lets you type in different descriptions for sharing on Facebook but on this one website it is being ignored on here.

I added some code to the functions.php file to stop Jetpack using it's OG data so the SEO plugin would have priority. It hasn't made any difference though.
I've removed the trailing / on the permalink - no difference

This bit of googling mentions the header file and says this is missing when someone else had the same problem, it's the description that is missing on mine though

<title><?php wp_title(''); ?></title>

https://wordpress.org/support/topic/...rking-properly

I don't want to go messing around with the theme because I really do not know what I am doing here.

I've spent literally hours on this and can't work out what is going on! Any ideas much appreciated.

did you try shutting off all plugins but that one? there might be a plugin conflict.

all apart from jetpack, menu image and the multilingual plugin have been disabled.
I've tried in a default theme too and it's doing the same thing, so it's nothing to do with Atahualpa.

argh! so frustrating. I have another site in French with almost exactly the same set up but I haven't updated that site with the latest WP (4) yet in case it does the same thing.

So disable jet pack and try

Jetpack is the thing that is making it feed to Facebook automatically so if I disable that it won't work to check it :-)

I'm going to wait to see if the next update to WP makes a difference I think.