I now have my VMG8924 working, and I think it was just fooling me all along!
On the GUI 'Status' page it still shows as having no IPv6 address even though it does actually have one.
Because of this I hadn't bothered to run any further tests as I assumed it would be pointless.
I then looked closer at the LAN setup and saw that 'Delegate prefix from WAN' was showing an IPv6 address with a /56 after it on the VDSL interface, and that got me thinking that perhaps it was getting an address - and sure enough it is.
My VDSL WAN is configured as attached ('Apply as Default Gateway' doesn't make any obvious difference if selected or not).