Well, daughter took car into the dealer yesterday for the airbag recall and to investigate the noise.
She rang me last night to say the dealer said they couldn't hear it!
I called the service manager this morning and asked if she dropped in could he have a listen. He agreed, no problem, and she went in at lunchtime (was passing anyway). He sent a tech out who heard the noise, looked at the car and pronounced that it's corrosion on the front discs and suggested replacing them.
I can't say it isn't that, but quite why it produces a quite loud "grabbing" noise (as if reversing with the handbrake not released) and then a ringing noise when going forwards is baffling to me.