We report angle-resolved infrared reflectivity measurements from a substantially sub-wavelength thickness, textured, metal-dielectric-metal microcavity. The two-dimensional surface texturing causes the structure to support flat-band, surface plasmon modes exhibiting efficient diffractive coupling to free radiation. Additionally, we observe a transverse magnetic mode that is due to phonon absorption within the dielectric spacer layer of the structure. The nature of these electromagnetic modes, their mutual interactions and the device band structure has been characterized by numerical modelling of the experimental data. With the exception of the phonon mode, the analyzed modes show that absorption of incident radiation predominantly occurs in the metal layers of the structure and at frequencies dictated by the geometry of the patterned top surface.