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?

There's a fair load on them.  They hold a foot bar on the side of a skiff.  I agree a bonded fastener would be best but there's no access to the back.  It was this reason why I thought an M12 slug would be good.  The thread is fairly course so should be ok in the laminate.  I thought the helicoil might also work but the thread is obviously much finer (M5 thread required).  It also means drilling a small hole so might be a first try before the slug.