[Closed] Cannondale eccentric bottom bracket adjustment???

I recently bought a Cannondale trail singlespeed with an eccentric bottom bracket.
I've swapped the rear sprocket for a smaller one, so need to take up the tension.
The manual says use a 4mm allen key in the 2 little holes, but a 4mm is too small on mine.
A 5mm fits in nicely, but when I turn it nothing happens and it's getting to the point where the allen key is bending.

I've tried turning it both ways incase it's a reverse thread, but no joy.
It's only done 40 miles, so it's not corrosion.

What am I doing wrong, do I need a special tool?