Done!
Eventually I used some reasonable-quality (Bosch) cheap carbide tipped hammer action masonry drills, and reground them with a small diamond wheel, of the type sold for cutting new slots on woodscrews, in a Dremel.
Zero rake, and double ground/split point, but I held the bit on one hand and the Dremel in the other to do that, so nothing fancy (I'm also a bit blind at those sizes without magnification, and I did it largely by feel and angle
- and I ain't got many fingers, or much feeling neither. so it wan't that accurate).
The carbide seems much better than any of the fancy metal drills - this also seems to be in general true of lots of masonry drills - and patience seemed to be the watchword, though I abused them at least as much as I had abused the expensive Fhrhei drills which broke in (very) short order.
This was for a 4mm (when reamed) x 80 mm (eventually) deep hole in Inconel X-750 - the larger Frhihe drills worked a bit better than the smaller ones (but still not worth it for Inconel IMO, the carbide cracked and seemed to be of lower quality, I'd stick with masonry drills).
Thanks all.