The 3D-echo-DLS (dynamic light scattering) flat cell light scattering instrument (3D-echo-DLS-FCLSI) presents the possibility of measuring slow dynamics of turbid and concentrated colloidal systems. It combines a modified 3D-DLS component and an echo-DLS component with the flat cell light scattering instrument. While the 3D-DLS suppresses multiple scattering, the echo-DLS allows measurements of slow dynamics or even on non-ergodic systems. The advantage of the thin flat cell is that it increases the transmission and reduces multiple scattering; i.e., singly scattered light that is required by the 3D-DLS is still available from dense turbid systems. In the first part of this contribution the 3D-echo-DLS-FCLSI is introduced and the instrumental performance is presented. The second part of the paper is concerned with the ageing behavior of dense fluids in a flat cell, and with confinement effects. Here, we show that ageing is strongly influenced by the process of filling of the flat cell. In some cases complementary methods can be utilized to measure special properties of the system; e.g., the multispeckle method is most appropriate for measuring heterogeneity effects. In the last part of the paper we compare glass transition measurements of an index-matched emulsion carried out using the 3D-echo-DLS-FCLSI and using the multispeckle instrument. We still find an α-relaxation in the glassy state.