I've got some ultegra levers that have a bit of play (5mm or less) that are starting to bug me, is there any way to take up the slack? to be fair they went under a bus before i owned them so that may have played a role 🙂

you sure it's the lever & not sticky cables ?

try pulling cable back by hand & see if there's still slack

I had a similar problem.

Solved with some new inner cables and a cheeky squirt of GT85 through the outers before installation.

Job done.

replaced the cable and outer today, there is a slight amount of movement before cable is engaged. Not the end of the world but a bit annoying

(bullshitting now:)
Try holding the brake caliper under slight comperssion while you do up the cable stop ?
And make sure the routing is nice & direct