Howden's bathroom thumbturn - how to remove?

[honestworker]

Hi. Some time ago, a joiner fitted a new (primed) bathroom door and a thumbturn for privacy - probably Howdens. I need to paint the door (with acrylic water-based?). I have not removed this type of privacy knob before - only the the type that is integral with the handle.. I have slackened the allen grub screw - nothing moves. I have tried levering off the round cover around the inside knob, but it moved only slightly. I do not want to use force and then damage the mechanism or the door surface. Can anyone please explain how I remove this thumbscrew without marking or damaging the door? Thank you in advance.
Howdens website - thumbturn

 

[opps]

Image two, there is a slot to put a flat head screwdriver in. Then prise the cover off. Remove the cover and then you an undo the screes and any grub screw.