I've done a bit more digging and have found the IPv6 dropping in the same way on a very old HP Netbook with a Qualcomm Atheros wireless card. I have passed all this on to Dell Support. Their reply is:
"Thank you for the email. Based on the information you provide it seems that the issue is with the software since it is also happening on the HP device. I would like to inform you that we only support basic software and we have different department for advance software team. You can contact them through the details below."
The details are a £48 no-fix no-fee consultation with a Dell certified expert.
My response is that it is a 5-month old machine supplied by Dell. It is not my problem that the Qualcomm kit does not seem to be working correctly. It is up to Dell as supplier to fix the machine so it works properly or replace it with one that does. I have asked how we escalate so more news to follow!