Magnetisation reversal is a process that is of paramount technological importance, as well as fascinating scientists in the field. Despite its importance, only recently have scientists begun to probe the limits of what can be achieved. Driven by consumer desire for compact devices using large scale storage, the size and time limit of magnetisation reversal have begun to be unravelled by exciting experiments using large scale national facilities and state of the art modelling. In the work presented here we develop a model of a ferrimagnetic material parameterised on experimental observations. The key magnetic features are shown to agree well with experimental measurements and provide a basis for more complex calculations. Using time and element resolved X-ray magnetic circular dichroism experiments, model calculations of laser induced magnetisation dynamics are compared to experimental measurements. We present results of switching on the sub-picosecond timescale and conclude that it is possible to reverse magnetisation using heat alone. The conclusion that magnetisation can be reversed without a directional stimulus is scientifically intriguing and never before predicted. Confirmation of such a reversal mechanism using heat alone is verified experimentally, comparing well with the model predictions.