Jeanneau 49DS shower tray trap and plumbing

[Alita49DS]

The shower tray on my 49DS drains extremely slowly.

I have checked the pipes but not been able to access the trap. There is no way to get underneath it. There is a central screw and then a s/s ring with 4 screws on the circumference.

https://1drv.ms/i/s!AqLwtI1czSAlkfNvBW8yutarMpkXAA

The central screw I assume would just let the trap drop - with the devil's own job to recover it from the inaccessible void.

Do the other screws allow the trap to be lifted? Other Jeanneau boats probably have the same design.

Please only respond if you actually know the answer and, preferably, have dealt with the problem. Surmise is a luxury I can already indulge in.