Tony
You're quite right, these can be very difficult to remove. Try squeezing the two at the top first, just to break the grip. The actual cover is quite thick and you are unlikely to break it. If you look closely at the cover, you will find 4 small circles moulded in, which are the places to press to release.
You might be interested to know that Ring make a PIR version which is a direct replacement, but you may have to add an additional supply wire, dependent on what you already have