The study of magnetic thin films by Lorentz microscopy in the transmission electron microscope (TEM) is a well-established technique. However, in many instances, images are recorded with the specimen in its as-grown state or in a single remanent state following application of a magnetic field outside the microscope. Although such images can provide valuable information, they rarely produce insight into the mechanism by which the magnetic material reverses nor do they show the magnetic structures that form during the reversal process. Furthermore, it is not possible to determine, for example, the field range over which a reversal takes place. Given the importance of such elements for magnetic sensing and storage applications, there is considerable incentive to develop techniques which not only yield the magnetisation loop for an individual element but which reveal how the spatial distribution of magnetisation evolves as a function of field.