This study evaluates the addition of Acalabrutinib to current standard therapy of Rituximab, Cyclophosphamide, Doxorubicin, Vincristine and Prednisolone (R-CHOP) for patients with previously untreated CD20 positive Diffuse Large B-cell Lymphoma (DLBCL) requiring full course chemoimmunotherapy. All patients will receive one cycle of R-CHOP. Two thirds of patients (Arm B) will go on to receive a further 5 cycles (every 21 days) of R-CHOP with Acalabrutinib. Acalabrutinib will be taken orally twice daily continuously in 21 day cycles. One third of patients (Arm A) will continue with 5 cycles of R-CHOP. Patients will be followed up initially for 24 months and then for disease status and survival until 114 progression events have been observed.