I had the same probs on Jurassic Karp when I stripped it down.

I chopped up an old 13mm ring/open ended spanner and welded in a nice bit of 3mm stainless flat to extend the daylights out of that spanner....
or you could make one from any bit of flat bar that will fit into the beam and by simply cutting a 'slot' in the bar that will fit the nut and your away.....


Remember you'll have the re-install to do or future re-tightens to do so its worth making a half decent unit.