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.
They're also just a few miles up the road from me so nice and easy to get supplies and pop in for a coffee.