Observations from the Heliospheric Imagers (HI-1) on both the STEREO spacecrafts have been analysed to search for very long period large amplitude stellar variability, finding six new candidates. A total of 85 objects, mostly previously known Mira variables, were found to show convincing variability on time-scales of over a 100 days. These objects range in peak brightness from about fourth magnitude to 10th magnitude in R and have periods between about 170 and 490 d. There is a period gap between 200 and 300 d where no objects were found and this is discussed. 15 of the Miras in the sample are previously recorded as having variable periods and the possibility for these and two other stars to have undergone a period change or to be irregular is discussed. In addition to the six stars in the sample not previously recorded as variable, another seven are recorded as variable but with no classification. Our period determination is the first to be made for 19 of these 85 stars. The sample represents a set of very long period variables that would be challenging to monitor from the Earth, or even from Earth orbit, owing to their position on the ecliptic plane and that their periods are often close to a year or an integer fraction thereof. The possibility for the new candidates to possess circumstellar shells is discussed.