George Cole is Smiley and making a good fist of it. He would have been 100 this week, which is why they've scheduled this now. They've started with Call for the Dead, which precedes Tinker Tailor by a few years. The continuity announcer indicated that they plan to repeat other adaptations they have in the archives.
https://www.bbc.co.uk/programmes/m002b5v9/episodes/guide
I absolutely loved the much later adaptations they did with Simon Russell Beale as Smiley. If those are still available anywhere, I recommend them. (I bought the boxed set of CDs. May dig that out and work through them for comparison.)
For other Le Carré fans, this seems to be a very good resource for tracking down the various versions. https://spybrary.com/discover-the-best-john-le-carre-radio-adaptations-by-bbc-a-complete-list/