I've used these now (the Multi-Monti hex head version from Screwfix) and I'm
pleased with the result. The only drawback was that I had to use a small
socket-and-ratchet tool to get them in, and the final few turns took a good
bit of effort. In fact I had to withdraw my first attempt, which froze
absolutely solid with about half an inch still to go. I decided that
perhaps I hadn't drilled deeply enough so I lengthened the hole and also
used a blower to remove the dust and residue: the bolt then went fully home.

So thanks to everyone for this thread.

Bert