If you're hinging the clam, it probs best to leave the filler pipe assy as-is.
if you're having a fixed clam, then "simply" follow the filler pipe in about an arm's length and there is a clampy thing - undo it and the neck has enough flexiblity to reach the RS bodywork quite effectively. (almost like it was designed that way!)
undoing the rusty nut n bolt in that confined space at arm's length unsighted isn't easy though...
have fun!