I'm certain I'm on the right track here, but there's a small chance I'm missing something.
Here's a picture of my Bosch fuel filter...
The thread on the adaptor is a parallel thread, and a steel washer is supplied. Surely there is no way this joint will be fluid (petrol!) tight?
I will ditch the washer and source an O-ring to use in its place.
Any alternate opinions?
Cheers,
Glen