Thanks, I have ordered the round ones and will give them a go in a piece of scrap. I did have the same thought about the hex type that it may bite a bit better. At least it's a plan B.Also available in bzp mild steel & stanless steel,might be worth seeing if you can get the half hex version,it may afford more grip,not sure on the last bit,as I have never tried them.
Forgot a bit,they come in countersunk & non countersunk version,best establish which you have before buying.