I'd be happy to wait for another couple of days, until Wednesday. If you haven't been able to find an answer for me by then, do I still get charged?
I haven't tried that, because the noise occurs only when the car is in motion. I've just had a passenger listen for me, and the conclusion is that the noise is definitely from the sat nav area of the dashboard. The best way I can describe it is by comparing it to a plastic box inside another plastic box, being rattled around. Please also note that the noise comes and goes, and it isn't related to rough road surfaces. Hope this helps.
I've taken the car for a spin until the rattle started. Then I pulled in and took a look under the bonnet with the engine running. The noise had stopped. It does seem to be the case that the rattle occurs only when the car is in motion. I couldn't tell whether the alarm was where you thought it might be. However, I will alert my dealer to this possibility when the car goes in for attention. As I was driving the car today, the plastic-on-plastic knocking seemed to come from inside the dashboard.
Thanks. The Honda dealer in question has arranged for one of its "master technicians" from another dealer in the chain to look at my car, so they seem to be taking me seriously. I will certainly flag up the possibility you suggested. Thanks for this tip and for your additional advice.