I've just bought a new light for my bike (here it is).

I have to install this:



on here:



(as you can see, i've already had a go taking everything off).

I need to take off that circular-with-groove part (which originally held the "spring" to the U brake) so I can get at the reflector mount. I need to remove the reflector mount, and replace it with the mounting bracket for the light (first picture).

How do I remove that circular-with-groove part? I can't get a spanner around it! I tried removing the bolt from the back, but everything spun round (which is why the groove isn't horizontal).

I'd appreciate any help.