I could have given you a bum-steer on that one...
You can fit the filler on the lower part of the bumper, although not recommended, it is possible and LPG fitters will fit it there, but they do recommend a tow ball or tow ball bracket is fitted to offer some form of mechanical protection for the filler point....
Also with a bumper mounted filler, the filler point is connected using a flexible hose and not a solid pipe, so it flexes and gives in the event of a crash, whereas a solid pipe may split....
As you were soldier.