Actually, sounds like my existing Open Graph tags should work with Discourse. Lets see what iFramely thinks…
Well, works perfectly on iFramely, including the mobile representation.
For some reason I cannot fathom, it is now working on Discourse (tested on the MIT Forum). The arrangement of the data is different to other platforms, including the iFramely test, but at least it works.
Your help very much appreciated JammyDodger, thank you.