The block works no problem and I've tested the filter as working on a production site and a desktop moodle but it's not working on our main production site. (3.5.12+ for reference).
So it looks to me that there's something on our moodle site that's interfering with the filter. I've upgraded, checked debugging, changed themes, purged caches and even moved it to the top of the manage filters page just in case that had an effect, but nothing has fixed it.
I think I'm out of ideas, can anyone suggest what I should look at next to try and track down the cause of this issue? Thanks.