Take the little tube off the metering head (secured by two socket cap screws) and try to free it off.
Its like a spring loaded allen key in the tube, so should push down and spring back and turn in both directions easily. Take it off and soak it in WD40 overnight and then have a go at freeing it off. If someone has tried too hard to turn it, the allen key socket may have rounded. All you need then is a new little tube, not a full metering unit...