Well if you have complained to the head of the department then theres not much else that you can do really, maybe make sure that a lot more people go as well so that it makes more of an impact? Alternatively at the end of each semester at my uni we have to fill out assessment forms for each of our lectureres and seminar tutors and rate them out of 5 and give comments. Its based on these that the lectureres get bonuses and stuff and with consistent bad assessments can cause an inquiry if I understand correctly. If you have these then if everyone on the course rates him badly and explains on these why he is bad then something will be done about it. Even though it is at the end of the module, you can still ask for essays and exams to be looked over if something happens to him as a result.
However, I've just thought then that in my uni at least, even though the lecturer/ seminar tutor marks all of our essays, a selection of them go to a second marker to make sure that they have been marked correctly so maybe he isn't all that bad. To you they may seem like random marks but maybe there is reasoning behind them. Maybe ask for feedback on your essays to see where you have gone wrong and were others have gone right? Also, I dunno what they do at your uni, but most modules 2nd and up have their exams marked externally.
So yeah, what I'm saying is that maybe he isn't a bad lecturer in regards to marking. But you can always appeal if you feel as though you have an unfair mark.