That line looks very much like the main line into London from Luton.
As such I would expect a 24 hour 7 day a week service and also unlike most of the rest of the network it also operates through Christmas as well.
Just checked Realtime trains Advanced for Harpenden and selected 0000 - 2359 for time scale this gives all trains through the 24 hours.
There are more than a few.
http://www.realtimetrains.co.uk/sear...=all&order=wtt