The aim of this clinical trial is to see if the drug called AZD2014 is effective and safe to use to treat patients with relapsed or refractory Diffuse Large BCell Lymphoma (DLBCL). The trial will also be looking at combining the antibody (Rituximab) with the drug AZD2014 in a small number of patients to see if this can be done without increasing the toxicity. 36 patients will be recruited to the trial. 30 will receive AZD2014 alone and the remaining 6 will receive AZD2014 plus rituximab. AZD2014 will be given as a 125mg tablet that is to be taken twice a day for 2 days out of every 7 (i.e. on days 1 and 2 of every week). Rituximab will be given via IV infusion on day 1 of every 28 days (once every 4 weeks) for a maximum of 6 cycles.