A sub-millimetre excess has been detected in three symbiotic Miras (AGB + possible post-AGB binaries, two of which are also symbiotic novae). The excess is probably caused by emission from dust at lower temperatures than the 300-K dust shell which dominates the near- and mid-IR emission. The cold dust was predicted by earlier modelling of the mid- to far-IR IRAS data, and it could be evidence for a planetary nebula stage in the evolution of the Mira's hot companion. Alternatively, it might be a relic of repeated symbiotic nova outbursts, or it may have steady-state origins in the winds of the hot or cool components. Ground-based telescopes can only measure the continuum level at wavelengths longer than 350 microns, and IRAS gave only upper limits at 100 microns, so current estimates of dust masses and temperatures are very poorly constrained. Our observational objective is therefore to measure the mid- to far-IR continuum using PHT-C so that we can disentangle the IR and mm continuum processes (dust emission; optically thin free-free) and thus rigorously and quantitively test the reality of the cold dust components. Accurate photometry in this region is crucial in order to model the dust emission and thereby constrain the temperature and mass of the various dust shells.