A main bearing housing for a wind turbine comprising a bearing arrangement having a sump located in a floor region of the main bearing housing. The sump comprises a floor pan surrounded by a side wall and includes an overflow arrangement configured to permit fluid to spill from the sump. The overflow arrangement comprises a spill passage configured with a spill inlet at or near the floor pan of the sump and a spill outlet located in a position between the spill inlet and an upper edge of the sump side wall. An advantage of the invention is that since the spill passage is fed with oil from a position that is close to the bottom of the sump, debris and sediment at the bottom of the sump tends to be entrained with the flow of oil and so tends not to collect at the bottom of the sump. The lubrication system therefore is able to clean the oil more effectively because the debris and sediment is encouraged to circulate around the lubrication system.