Yes Bob, what you're saying does make some sense and will make even
more after I've read it a few times. So does Roy's.

I've got a pretty good handle on the basics, having a physics degree
and a long interest in hobby electronics. But as black arts go, the
workings of antennae and tuned circuits come pretty high on the list!

Some experimentation is obviously needed. I'll start with a simple
dipole and a run of coax, the outer screen attached to chassis. How
does that sound? Is it worth buying a proper aerial or, being in the
loft, would a home-made rig suffice? What would you suggest
design-wise?

Money isn't really an issue - just that there's no point in buying kit
that turns out to be unnecessary. As I said, the signal is already
improved by hooking up the flying aerial lead to the wire in the duct.

Meanwhile I'll let you guys fight out the theory!

Many thanks for your advice

Paul