Hi. I've run a cable from a socket in the rear luggage compartment for an electric winch to the bulkhead. For the life of me I can't see an easy route to get the cable through the bulkhead to a fusebox I've fitted next to the battery. Has anyone else managed it and how?

You'll need to find where other wires/cables go through, and feed the cable through the grommet with them.
A straightened wire coat-hanger with the end smoothed off (to avoid wire damage) can be poked through the grommet alongside the existing wires, then tape a thin cord to the end and pull back through.
Then tie/tape the cable to the cord and pull through.
How thick is the cable by the way?
If it's too thick to pull through an existing grommet, you may have to drill another hole for it, and give it it's own grommet.