First snag! Front fork removal

Well, I've hit my first problem. With the front wheel removed, I drain the fork:

and then attempt to remove the bolt that holds the suspension unit into the fork. To stop the body rotating, I screwed a spanner into a brake mounting point:

But no matter how hard I try, including having someone else help by holding the spanner really rigidly, this screw will not turn. I tried the same thing on the other side, same result. Am I doing something wrong?

Advice is very welcome!