A litre a month isn`t to bad so don`t panic but there is a problem. The EGR cooler has been known to get leaks but most likely it`s the head. As Mike Foster suggested you can try a few bottle of K seal as you might be lucky and fix it that way depending on where the leak is. most heads crack between the valve ports which K seal will not fix as it is under high pressure and the crack will gradually get worse. I used to sale re-con heads and other engine parts but don`t both now. If you buy a secondhand head make sure it has been pressure tested first as fitting a new head costs more in parts and labour then the head itself.