I've been asked to fix a friends boat where some self tappers have pulled out over time.  I have access from 1 side only so no bigheads I'm afraid.  Its 10mm thick GRP.  Can I use a helicoil in it?  Or should I thread it out to M12 and bond in an M12 piece with an M5 tapped hole down the centre?  Is there anything else I can do?

I've got the Memfast kit at home, its fantastic ( I bought all the thread sizes I could afford at the time!) I used it to build the kit car but have used it loads of times since on other projects so it was well worth the purchase. It works really well in GRP but make sure you select the correct rivnut for the job.