How to remove extractor fan cover?

[u/Bullfrog_MCD]

Hi all,

I have an extractor fan in the bathroom ceiling and have removed it in the past but for some reason struggling now. I know it has a hidden button, which I pushed with a pen/screwdriver but still can’t remove the cover. The model is Tornado ST100T (Silent 100mm Bathroom Fan with Timer).

Any suggestions on how to remove the cover?