AVB Fuel filter

[Vagrant]

I haven't been on here for a while but here's a question has been bugging me amd I'm hoping someone on here has encountered it. '04 estate with an AVB engine.
When I got it I changed the fuel filter and it leaked at the side of the plastic valve.
Three fuel filters, two valves, a couple of sets of O rings and it still leaked. I took the two pipes on the valve and joined them with a piece of 8mm copper, all good, no leak. It's been running like this for 20+k miles, but why?
I tried priming the filter, not priming and I cannot understand it - I'd rather have the car as it was meant to be - I doubt VW put it there without a reason!
Am I missing something really obvious?
Ivan