Thank-you for all of your responses.

I have taken the car to a couple more mechanics, all of whom seemed to
be convinced that the knocking comes from some play in the
rotor/wheel-bolt assembly. With the wheel off, I can grab the wheel
bolts as a group and rotate them about a degree before they bear
against their housing with a metallic clank. This might be a bit of a
red herring though, as I have just wound some paper into the gaps to
remove the play (which seemed to work) temporarily and the clunking
remains.

The rotors sound like quite likely culprits, as you have all
suggested. Particularly as the sounds happens more rapidly at high
speeds, implying that it is to do with a part that rotates with the
wheels. Now the real challenge is to work out which rotor the noise is
coming from - it?s very hard to tell when you?re inside the car, and
it seems to be impossible to create the sound with the car jacked-up.

Thanks again for your help. I?ll keep you posted of any developments.