Apologies for the delay. I've just been through the entire checklist system, looking at every control I can find. Nothing obvious pops out. I did have a set of screenshots but couldn't get in into 100kb to upload (that's way too small). I've got a set of users in the module but it still shows nothing in the block. I could see no control on the block when the editing was on to actually add users or check if they were there.
Any ideas where I go from here? Happy to try almost anything because it's a very popular idea.