If the base of the bracket is loose you can tighten it with the proper size of ball-end allen wrench. It has to be the ball-end type because you won't be able to come straight at the screw. When you tighten the bracket make sure to check the intonation. The bracket should have a slot rather than a round hole that allows for some adjustment.

If the bracket is tight but the lever itself is loose you will need a proper size of open end wrench to tighten it. There is a spring washer that applies a bit of pressure. Adjust the lever to be as tight as you like it. It has to move when you want it to but not move when you don't want it to.

These recommendations for Loveland levers and written in American, i.e wrench=spanner (I don't know the UK translation for allen wrench)