Hi Fizzer, I recall someone else had a similar issue,
Are you still using the original t 30 head bolt, does it still screw in all the way or has it cross threaded turning the insert ,
It is fiddly to line up, I replaced the bolt with a thumbscrew as it was so fiddly to line up I could see cross thread issues and didn't want to use tools, Ebay sellers do something similar,
looking at the photo drill into the as viewed bottom area of plastic and up into the insert to allow a machine screw to be tapped into the hole securing it,
or a self tapper worked in with a bit of thread lock on after first screwing it in to cut a thread,
I expect the insert is larger on the inside to prevent it pulling out ,so just stopping it turning and cleaning up the threads should do it,
I didn't use the off the shelf replacement tool less e bay one , I used a plastic knob with 6 mm thread,
I needed a longer thread as I use a hepco Becker mini rack and givi 33 l top box, which needed a spacer in the seat bush where the bolt mounts to lift the seat to clear the brackets,
Roamer