Success, I bought a VR600 v2, 2nd hand off ebay (like new but half the price), updated it to the latest firmware from TP-Link's website ,and IPv6 is now working. Although it didn't want to know with the installed firmware which was reported as up to date by the router so an hour wasted sorting that out but c'est la vie.
The VR600 connected instantly and is synching a smidge faster than the Zyxel, both using a Broadcom chipset.
Hopefully, there is now no way that BT can mess with my router and fortnightly reboots will be a thing of the past.
