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?