Sally Ann was only popping into the lab as a favour to us while on the island doing something else and she was keen to point out that these type of specimens were not her speciality so please take anything below as preliminary and only based on a quick look at our images. Understandably Sally Ann couldn't get us down to species level on a quick trip but she was able to identify the families of each type of arthropod we'd found.
The largest ones, easily visible to the eye, were the millipedes (main picture above). We'd always assumed that Arthur Pod and his cousins were millipedes but Sally Ann also identified another large and obviously segmented animal as a flat backed millipede which was new to us. Millipedes are leaf litter feeders and don't burrow into the soil so their presence within the hoard itself would seem to prove that they got there at the time of the burial. (We know they're not recent as they are all stained with the copper corrosion from the coins and only their hard exoskeletons survive). Presumably they were on the surface of the soil dug for the hoard's hole and were thown on top of the coins as the same earth was later shovelled back into the pit.
Most of the animal remains we found were much smaller, only a couple of millimeters long. There were broadly two types, one looking like a centipede and the other more beetle like. It turns out both are the spledidly named pseudocentipedes, also leaf litter eaters but which dwell in soil down to depth of about 50cm, somewhat above the depth at which the hoard was originally buried. Once again they were probably thrown onto the coins in the backfilling process. The very smallest animals we've found, hardly visible to the eye at all, are elongated bodied springtails, once again surface dwelling leaf litter eaters.
Finding out all this has been great for us and so a big thank you to Sally Ann. I'm sending all our bug images to her soon so she can have a better look and perhaps share them with colleagues, so hopefully we may yet find out more.