Neutron scattering and AC susceptibility measurements have been made on isotopically enriched 1 5 4 SmMn 2 (Ge 0 . 9 Si 0 . 1 ) 2 . A temperature versus pressure magnetic phase diagram is proposed. The incommensurate cone structures detected in SmMn 2 Ge 2 at low temperatures and pressures do not appear in the phase diagram of the Si-substituted system. There is no evidence of a change in the magnetic structure associated with a small peak in susceptibility observed at around 145K which has previously been interpreted in terms of changes to the cone structure. However, as with SmMn 2 Ge 2 , the non-collinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (001) ab-planes in the ferromagnetic states than in the antiferromagnetic state. With increasing Si substitution, the antiferromagnetic phase becomes the magnetic equilibrium state over a wider temperature range. The observed changes in the Mn-Mn spacing and magnetic coupling are largely consistent with previous related observations, although under a pressure of ~8kbar, low-temperature magnetic structures are found below the critical lattice spacing of a=4.031 9 which are similar to those observed at ambient pressure.