Our sample of fully oxygenated and de-twinned YBCO has previously given the first measurements of the intrinsic vortex lattice in this material, up to fields of 11 T. We have witnessed a series of field dependent vortex lattice structure transitions, which we believe are due to the anisotropic superconducting properties of the basal plane. We can modify these anisotropies either by going to high fields, which appears to suppress the CuO chain superconductivity, or by rotating the field direction away from the c-axis. Our new 17 T magnet with rotation stage will allow us to do both as a function of temperature, giving a great deal of new information about the Flux Line Lattice and hence the structure, field- and temperature-dependence of the superconducting gap.