Gavin, if the kit I posted above is suitable for your wand, your trigger valve will have a nut which you unscrew to disassemble.
Inside will be the little round thing which is a spacer, and the flat end of the valve stem sits against it. The valve stem pokes up through the valve body, and the rounded end is the bit which sticks out - that is what your trigger pushes down upon when you operate the wand.
Replace with the new spring, as they wear over time. You might even find your current spring is broken but still partly functioning, that can cause drips. If your old spring is still in one piece keep it as a spare.
As Joe says, order at least 2 so you have spares, only takes 2 minutes to replace valve assembly but you'll look a plonker if you can't finish a job because of such a trivial thing!