It's going to be a talking heads doc with archive rather than an actual 'reunion' episode of the show. I believe it to be currently titled 'The Story of SM:tv'. H & Claire have recorded their bits for it already, which they both mentioned on H's radio show here: https://www.youtube.com/watch?v=fad43liRPt4.
It could be that they’re planning both? A retrospective talking heads doc to air the night before a live reunion show? Or something like the Big Breakfast did in their final ever show where they featured a 30 minute retrospective (“I LOVE THE BB”) as a feature within the 3 hour long main show?
They’ve made such a big deal about actually coming back and doing a live reunion episode on a Saturday morning in the past that I find it hard to believe it would just be watered down to a talking heads show - even if plans have had to be delayed and changed due to both COVID and McPartlin’s situation when it was originally planned.