Vauxhall Tigra "clutch" mechanism on convertible roof not engaging

Found from an archived post that was never answered:

"When I press the button to open the roof, the "clutch" mechanism which opens/closes the black plastic "shelf" by the boot sticks/doesn't engage. The roof quite happily goes into the boot or comes back up but the "shelf" wont close and makes juddering sound. As the operation is incomplete the windows don't automatically go back up and this shelf and its "side wings" stay open. A friend has diagnosed the problem - it's the "clutch" mechanism which should engage to open close this shelf and is slipping rather than engaging. Does anyone know if it can be adjusted or whether I need a new one please?"

I'm having the exact same problem with my Tigra - the clutch mechanism doesn't seem to be catching/engaging, so the process to open the roof won't fully complete. I've looked through the little hatch in the parcel shelf behind the seats up into the mechanism, and seen the clutch makes a spin, attempts to catch, spins again, attempts to catch - rinse and repeat. It makes a vrrr-clunk-vrrr-clunk noise! Anyone have any wise ideas if it's a tweak or replace jobby?