Within the literature significant variations in martensite start temperature (Ms) can be observed for samples of identical composition. However, the factors affecting Ms are not well understood, limiting industrial uptake. Recently, claims that the isothermal variant of the ω phase (ωiso) may be driving many changes in Ms have been called into question due to the absence of any compositionally distinct ωiso following thermal cycling. In the present study, in situ synchrotron X-ray diffraction has been employed to conclusively show that dramatic changes in Ms can be replicated without any formation of ωiso. Dislocations have been shown to be mobile at the temperatures reached and, as such, an alternative mechanism based on the total stress model has been proposed. This added understanding can rationalise many of the discrepancies observed in the literature, and ultimately improve industrial uptake of this class of material.