Thanks for your comments.
This movement is not "flex' in the sprocket, it's movement of the whole cassette. Interesting that so many new cassettes in the bike shop moved Sekine! That's what I find too, the works bike assemblers do seem to allow the cassette a mm or two of movement.
However, If I was to try to remove the movement...
Since my wheel bearings are correctly tightened, and there' no play at all in the wheel itself, it seems there's 3 possible places the slackness wobble could arise.
1) Loose cassette lockring, not correctly seated and tightened to 40Nm. So the cassette moves on the splined freehub body.
2) The large hollow Allen lockbolt inside the hub that anchors the freehub body to the main hub is loose. What Allen key size is this in Formula hubs?
3) The freewheel mechanism inside the freehub has too many (or not enough) shims inside and needs adjusting.
Is this a correct assessment?
Of these 3 my guess it is the last one because I saw the lockring tightened correctly, and it's unlikely the hollow hub lockbolt would work loose. I suspect we will find the hub lockbolt is already fully tight and any play arises from within the freewheel mechanism. So I'll need to have shims ready.
It's a 40km ride to the bike shop so I'd like to prepare in advance for the possibilities.
Once the cassette is removed, how do I distinguish 2) from 3) if there's still 1-2mm play on the freehub? Does the hollow lockbolt have to be torqued to any particular setting, or is it just tightened up until there's no play?