Triple-negative breast cancer (TNBC) is a subtype of breast cancer that lacks the expression of oestrogen (ER) and progesterone (PR) receptors and lacks overexpression or amplification of the HER2/NEU gene. TNBC accounts for approximately 20% of all breast cancers. The development of the metastatic disease is more common in this subtype, indicating a poorer prognosis with worse overall survival compared to other subtypes. Radiotherapy has long been used a cancer therapy and is known for its direct toxic effects on tumour cells. There is an increasing body of evidence supporting the idea that radiotherapy may be useful in stimulating the immune system so that immunotherapy may be more effective in treating widespread disease. Researchers are aiming to investigate the optimal dose of radiotherapy to stimulate the immune system, which is the purpose of this study. The participants of this study will be treated with either a single fraction Stereotactic Ablative Body Radiotherapy (SABR) then Atezolizumab (Arm 1) or a 3-fraction SABR then Atezolizumab (Arm 2). The aim of this study is to compare the different SABR schedules (Arm 1 and Arm 2) and which schedule is more effective in treating TNBC.