Thanks to everyone for your replies.
Just to clarify, it's not a standard fixed screwdriver with a hex bolster I'm bemoaning the rarity of but driver handles that take bits and also have a hex bolster.
Jason, yes, I tend to use 50mm bits as standard these days. One drawback with having to put a spanner on the bit itself is that you're quite close to the screw head, and so room for a spanner can sometimes be an issue. There are of course considerably longer bits than 50mm out there, but long ones aren't as easily available at reasonable prices as the shorter ones.
The other slightly less than ideal thing is that a 1/4" hex is a rather small AF size to be winching on by hand with a spanner. This is no doubt why many of the screwdrivers you see out there that do have hex bolsters have shafts that are hexagonal in cross-section anyway. If the shaft's hex was felt to be adequate for putting a spanner on why would they give you a bolster as well? I've got some old Gordon screwdrivers with square section shafts and these don't have bolsters but the shafts are usefully thick at, I think, 3/8" and 7/16" AF.
Martin, I wasn't aware of the existence of the Dewalt, so thanks for that. I recognise the handle on it, and, if it's the one I'm thinking of, it's substantial and grippy, so I might give it a go.
The only other bolster bit driver I know of is the Wera Vario 80. Very unhelpfully, though, this takes 6mm hex bits, not 1/4". I'm also not keen on the Wera bit driver handles generally, as they're inconveniently small when compared with their standard Kraftform screwdriver handles on, say, PH2 and PZ2 drivers.
There is one other way of adding torque when screwdriving with a shaft-shaped object and that is to use one of these with a bit adaptor on the male square drive and a ratchet or breaker bar in the female drive in the end of the handle. The only drawbacks with this are that, again, the handle is too dinky for me and it's not as ergonomically satisfactory in my view adding torque to a screwdriver at 90 degrees to the shaft from the far end of the handle as it is doing so further down where the shaft begins.
I get by with what I've got, yes, but I do wonder why standard screwdrivers with bolsters are reasonably common whilst bit drivers with bolsters are as rare as pins in puddings.