basically with the experience I've had with my demon, even though the QEV looks good I'd still change it, the amount of problems those little rubber things cause is unbelievable. also when taking the marker apart and putting in back together i sometimes knock an oring of the manifold and causes a leak.
as you've said you have not worked on a demon before, i still suggest buying a large spares kit, if you manage to fix this by yourself you'll get to know the marker better and be able to fix it easier in the future