A panel comprising: a frame member (70) defining a periphery of said panel; a first wall (77) retained by the frame (70) and a second wall (79) opposing said first wall (77) and together with the first wall (77) defining an internal space (73, 74) of the panel; the panel further comprising at least one intermediate wall (78) disposed in said internal space intermediate the first (77) and second walls (79) and which creates a first space (73) in said internal space between said intermediate wall (78) and said first wall (77) and a second space (74) in said internal space between said intermediate wall (78) and said second wall (79); characterized in that the frame includes respective abutment surfaces (80, 81, 82) which receive and retain respective first, second and intermediate walls wherein; the intermediate wall (78) insulates said first wall (77) from said second wall (79).