Study to Learn if 200mg Test Drug (Fostamatinib) Helps People With Large B-Cell Lymphoma,a Type of Blood Cancer

Inclusion Criteria: * Aged at least 18 years of age. * Patients with relapsed or refractory diffuse large B-cell lymphoma who have previously received R-CHOP (or equivalent) chemo-immunotherapy and high dose chemotherapy with stem cell rescue, or who are ineligible for high dose therapy with stem cell rescue. * Measurable disease as defined by Cheson et al 2007 criteria. * One fresh pre-treatment excisional or core needle biopsy from suitable and accessible site. * World Health Organization (WHO) performance status 0 to 1. Exclusion Criteria: * Treatment with nitrosurea, mitomycin C, investigational agents or study drugs w/in28 days of first dose of study treatment, any other chemotherapy, immunotherapy or anticancer agents w/in 3 weeks of first dose of study treatment, previous fostamatinib. * With the exception of alopecia, any unresolved toxicities from prior therapy or surgery greater than Common Terminology Criteria for Adverse Events (CTCAE) Grade 1. * Uncontrolled hypertension (defined as \>140mmHg systolic and/or \> 90 mmHG diastolic at baseline with or without antihypertensive therapy. * Evidence of tuberculosis (TB). * Inadequate boen marrow reserve.