Trouble with 105 LH double / triple shifter

Brand new left hand shifter now on bike. It even has "double" printed on it.

But (and there is always a but with this bike) the shifter has more float than a scoop of ice cream in a glass of coke*.

Using the larger lever, when shifting into the big ring or braking, there is no distinction between the two planes of movement - it appears to (try to) shift and brake at the same time, unless I use another hand to steady the movement.

I'm booking it back into the shop, but wouldn't mind having a go at fixing it first - Any ideas?

• I've been saving that one up for a while now.
  
  

The broken ones I've dealth with simply didn't shift down (once in bigring). So if they are broken its a new one on me.
I wouldn't try repairing it personally. If you fuck it up you're no better off.