1. In a radiotherapy system comprising a source for producing a beam of radiation and an imaging device for imaging from the beam, both mounted so as to be rotatable about an axis, and an imaging apparatus having a volume within which imaging takes place, a method of aligning the beam using a fiducial phantom between the source and the imaging device, the method comprising: rotating the beam and imaging device in a trajectory about the axis while obtaining a plurality of images of the fiducial phantom from a plurality of different angles, wherein at least one image includes a feature of the imaging apparatus, adjusting the trajectory of the source relative to that feature so as to position the isocentre substantially centrally of the volume, determining from each image of the fiducial phantom the position of the source at the rotational position the image was obtained, and calculating the centre of rotation of the positions of the source to define the isocentre of the system.