Has anyone ever found an efficient loft antenna - one that really works on
all bands ?

Hi John, Any antenna that is efficient on all bands outside your loft will
perform about the same if you put it inside your loft. (maybe not verticals)
There will be some changes due to the metal that will be present in your loft,
but the efficiency will not suffer much.
The obvious problem is how to stuff the antenna into the limited space. The
most efficient all band antenna that I know of that could fit is a 130' doublet
fed with ladder line and a tuner. You may not have 130' available so you
reduce it to 100' without much efficiency loss. You could zig-zag it to reduce
the space needed even more.
This gets me to what I did. I have a 2 story house with a 40'x30' attic. I
put 100' of wire up there, and fed it with ladder line and a tuner. Works well
on 80m-10m. Works all the electronics in the house quite well.
73 Gary N4AST