It's too clean fer one thing . . . . .
As the adapter nut is of British manufacture it's not Metric or UNF (SAE). All British oil line fittings and fuel line fittings are Whitworth based. The correct socket is 13/16 W. I ordered one off of Ebay several years ago. As most people don't have the correct tools, the nut is often "buggered up" and may require some hand filing to repair. While at it, it's a good thing to check the underside where it seals against the fibre gasket. Often using the wrong tool causes burrs which the gasket can't take up. I usually replace the small fibre gaskets with nylon ones sourced from Earl's as they are thicker and seal better.Didn't you see the 36mm wrench I had for sale at the United. Cut short and a box end for the top with a cut out to fit over the metal hose end. Then all you need is 3 hands and some luck.
My best was 3 times several cuss words and several hairs pulled out before it stopped leaking.
Dave's right the stupid band clamp cannot tighten enough to fit a Fram or a Wix or the Autolite FL-1A and keep it from turning.
3 joints is two too many but good luck. Look over the previous discussions. One suggests " I'll take the TEAE 2 point deduction for good functionality. " Paul F. " I have to go back and like that posting.
Have had Whitworth tools forever!! taps and dies too. MY dad had had Sunbeam Talbots since the late 50's. I restored a '66 Triumph T100 (Tiger) about 15 years ago. Found a Honda product to seal the case. Leave it to the Japanese to figure out how to stop British bikes from leaking!