how do i change an in-line fuel filter?

IIRC these use push-on hose fittings. Make sure they have been pushed fully home.
If the person who last changed it prised it open with a screwdriver instead of using the proper tool the fitting may be damaged, in which case you may have to cut the fitting off, push the hose over the filter ferrule and fix it with a jubilee type hose clip.
Check that a proper vauxhall filter was used, cheap copies may not seal properly.
Observe all usual safety rules when working with fuel/under car.

spud
September 2009