This proposal follows from a short previous test experiment (RB 920345) aimed at understanding the nature of local structure distortions in YBa2Cu3O6.9 and their possible role in superconductivity itself. The neutron PDF data we collected in that experiment are dramatically higher in quality to that used for previous PDF studies, and we see this as one key reason to justify a "proper" PDF study such as we propose here. Our RMC analysis also shows evidence of correlated distortions in Cu-O bond lengths that is likely to have a strong effect on the electronic properties of the material. We are seeking here to carry out a neutron total scattering study of eight compositions within the YBa2Cu3O7-x family so that we can correlate changes in local structure with superconducting behaviour (e.g. Tc).