Managing synergies and conflicts between voltage support services and network losses is essential for the cost-effective integration of distributed energy resources (DERs). This study presents the results of studies investigating the impact of using DER reactive power services on distribution network losses. By using year-round optimal power flow analysis, a spectrum of studies on a number of distribution network areas in the southeast of Great Britain was performed to calculate distribution losses under different control scenarios. The studies demonstrate that the use of DERs to provide reactive services to the transmission system may increase distribution network losses. On the other hand, DER reactive services can also be optimised to minimise distribution losses. The studies also analysed the impact of optimising tap changing transformer settings on the distribution network losses reduction.