Rob, the expanding type unheaded inserts look like the ones for me.

I'd seen those so I don't know why I didn't mention those in the first place! I think it might have been because I didn't like the look of the non-expanding unheaded ones, so they were on my mind when I posted !

I've just thought, you could use those unheaded ones in counterbored holes, giving a more compact design than headed ones.

But the barbs on them do suggest they are for one-sided operation.

Anyway, i've requested pull-out data for use in delrin.