A new Aurivillius phase (generic formula M(2)A(n-1)BnO(3n+3)) has been synthesized with n = 3 and containing manganese, Bi2Sr1.4La0.6Nb2MnO12. The structure has been investigated by X-ray and neutron powder diffraction and found to be tetragonal (I4/mmm) at temperatures down to 2 K, with a = 3.89970(7) Angstrom, c = 32.8073(9) Angstrom at 2 K. There is significant cation disorder between Bi3+ (predominantly on the M sites) and Sr2+ and La3+ which prefer the A sites: 19(2)% of Bi3+ occupy the A sites. This disorder, leading to occupancy of M sites by Sr2+, is thought to relieve strain due to size-mismatch between the fluorite-like and perovskite-like blocks. A high level of order exists between Mn and Nb on the B sites, with Mn located predominantly (76.1(6)%) in the central B site whilst Nb preferentially occupies the lower symmetry, outer B site, where it undergoes an out-of-centre displacement towards the fluorite-like blocks. Magnetic measurements indicate that this material displays spin-glass behaviour on cooling. Synthesis of the Mn4+ analogue Bi2Sr2Nb2MnO12 was unsuccessful, possibly due to the small size of the Mn4+ cation.