Intestinal Microbiota Transplant Prior to Allogeneic Stem Cell Transplant (MAST) Trial

The goal of this clinical trial is to test the ability to restore gut microbiota to healthier levels in patients with blood cancers scheduled to have stem cell transplant. The main questions it aims to answer are: * Tolerability and acceptability of intestinal microbiota transplantation (IMT) versus placebo (as assessed via patient perspective questionnaires * Changes in gut microbiome diversity across all timepoints * Markers of general health, infective/microbiological and haematological outcomes including, days of fever, admission to intensive care unit, survival, non-relapsed mortality, and incidence of graft-versus-host disease across all time points measured. Participants will be asked at their routine follow up visits to, * Provide stool, urine and blood samples at the scheduled study visits * Complete questionnaires at selected visits * Swallow either Placebo or IMT capsules once at the second study visit which will occur 2 weeks prior to the stem cell transplant (+/-3 days) Researchers will compare IMT capsules and Placebo to investigate the change in gut microbiota diversity.