From the PH thread:
''On my last car, I had one of these sneakily hidden behind the fuel flap.
However, I have since found out these are not CoP-11 compliant (the UK LPG approval body) as you need to always use an adaptor which is frowned upon. You should also mount the filler so that it has more than a 50kg breaking strain in case you forget to detach the filler and drive off (as if!).
So I think I will just mount a conventional one to the tow bar, but set it back under the bumper''