Inter-/intra-scanner differences contribute to uncertainties in results from multi-centre longitudinal imaging studies. The Sequential Imaging Biofocussed Radiotherapy (SI-BiRT) trial is a multi-centre longitudinal imaging clinical trial that aims to develop imaging biomarkers using mpMRI to predict radiation therapy response in prostate cancer. Here we quantify the inter-/intra-scanner variabilities in the mpMRI protocol and scanners used in the SI-BiRT trial using commercial phantoms. ADC measurements were highly accurate while VFA T1 mapping overestimated reference values. ADC and T1 showed high intra-scanner reproducibility while R2* showed higher variability. The quantified uncertainties will be considered when interpreting results from the SI-BiRT trial.