A long socket or a tubular box spanner.
You need to flex the adjuster sideways a bit to slide the socket onto the adjuster. Remember, with handbrake off, it doesn't need to be done up tight, so flexing should always be possible.
Just one other point, one reason that handbrake cables need to be adjusted is that they are about to break! Take the driver side rear wheel off and closely inspect the outer of the cable where it bends to go down towards the brake drum. If the outer is damaged or split at that point, it allows water in and consequently the inner will rust, fray and break off. A common problem and real pig to replace.
