Charlie,
Actually, 8.4.79 is the latest for the iFly Devices. 8.4.81 is the latest for the Android. We typically will run a slightly different version for each platform to help keep versions sorted out here in the office. Plus, it helps us know what device your are on by looking at a version number.
Now, if you are on a Tablet, and only have 8.4.80, then I'd suggest you go the the Play Store and search iFly GPS, if you don't see an update option, go back and try to searching for it again. I've seen this several times before where the Play Store gets things confused on versions and a refresh or new search seems to straigten it out. Another option is to allow the iFly GPS App to Auto-Update, the will ensure you always have the latest. I hope this helps.