Constitutive behaviour of fusion-relevant steels in the post-necking regime is necessary for detailed design of future fusion device components. A Finite Element Model Updating (FEMU) procedure using Digital Image Correlation (DIC), a simulated DIC filter and novel constitutive law was developed. The effects of the DIC process and experimental noise on the uncertainty of optimised parameters were quantified using synthetic image deformation techniques. For large plasticity problems, the effect of measurement system noise was found to be negligible. The FEMU method was applied to small-scale specimens of Eurofer97 ferritic-martensitic steel of two thicknesses. Strain rate sensitivity was found to be necessary to explain observed post-UTS uniform strain due to delayed localisation onset. Local strain data from DIC measurements have been demonstrated to improve the accuracy of the identified constitutive model compared to global data alone. Additionally, DIC strain data can provide information to support the inclusion of additional model physics, such as rate sensitivity.