[Chris_Rogers]

>Thanks Chris, found them, looked at the connections, waggled
>them, they seem fine.
>It's a curious and very infrequent occurence. The light has
>never come on while the engine is running - when it does, it
>just appears on the morning start-up, although perfect the
>night before. Cancelling it or not makes no difference, and it
>never re-appears if I do cancel it, until the process repeats
>a few weeks later.

If the problem only occurs when cold starting it makes me suspect it's caused by low volts, maybe a loose connection somewhere or the ignition switch contacts.

>Supposing the connections are fine, does that point to
>something beginning to go wrong inside a module, or can the
>diagnostic system itself develop a fault?

The modules are very reliable I would doubt they are faulty I would have a look for poor fuse terminals, relay contacts or ignition switch contacts. Difficult with an intermittent fault, could be worth measuring battery volts on cranking maybe the battery is begining to fail.