The rabbit holes that I filled in are still filled in, so that seems to have been successful, but I found another one. This was carefully hidden in the hedge bottom and the pile of spoil that I thought had come from the hole I filled in, now looks more likely to have come from the newly found hole. I shovelled a good load of the spoil down the hole and we'll see if it gets dug out over the next few days. If not, them more cement will complete the job.
I took a quick look the hedge bottom along most of the north side of the allotment site and there are obviously other rabbit holes, only one of which has fresh-looking droppings outside of the hole. I don't know enough about the habits of rabbits to know if that is significant or not.