I've had this unit for several months, and in all that time it had detected nothing, so, after the big thunderstorm a few nights back during which it detected absolutely nothing despite the lightning being directly overhead, I finally went asking questions.
I learned that the unit had arrived with the DIP switches in a non-working position. The documentation is very unclear, but it turns out that the correct default setting is with all the DIP switches pushed down towards the battery compartment. Mine had arrived with them all pushed up away from the battery compartment, That looks reasonable when compared with the documentation (and the details on the back of the unit), but, unfortunately, it means that the antenna is disconnected.
I have now changed the setting to the correct default (with all the DIP switches pushed down towards the batteries) and my unit detected its first ever strike last night.