I had this untill recently with my RS7s and so did my mate before me, if the bb is not bent it will simply be where the whole bashguard attaches to the arms, there should be a lockring that tightens to the arm, seems to be a common problem, that has simply worked it's way loose. The bodge it method, required for a quick fix, is carry a hammer + screwdriver with u and slot it into the locking then give it a bash (giggidy) the propper way is take the drive side off and find the right tool ( a freewheel tool of somekind fits, I'll find which one for you in the morning) locktight the f**ker up, trap the tool in a vice and spin the crank arm around on it untill it is as tight as a new-born jewish baby, simples!