It is still most likely to be rogue software that has been downloaded to a device that is causing the issue. Provided that is that your wireless password is kept secure and has not been shared with anyone, then the likelihood of being 'hacked' through the router from the internet is almost non existent. The hacker would need to target you personally and directly. Most unlikely with a dynamic IP address. Then they would need Administrator access to your PC/laptop/MAC to be able to do much there. Without an account on the machine they won't get anywhere. So that brings us back to something downloaded by someone on your LAN.
Are you using good antivirus/firewall software? Several free ones are available for PC, MAC and Android devices. Search on Google.
A decent browser with add blocker and a script blocker will help, a little. Firefox and Opera are available for mobile devices. Though there is no substitute for common sense, never just click a link without knowing what it's actually going to do and avoid dodgy web sites. Like pirate sites and others that don't need named here.
Never click a link in an email that you can't verify as trustworthy.
As for the browser, setting it to delete everything when closed can help a lot, no info stored for the next session.
You could have a look online (Google again) for Trend Micro. They have a free service called Housecall. Basically it scans your PC, whatever for malware. Not used it since I worked in the PC shop but it cleaned out a lot of customers machines back then so should help. Certainly can't hurt.
As for ARP. Heard of it but never had any experience of it. Couple of links for you to check, loads more out there, (Google?). From the sounds of that it's becoming almost certain something was downloaded to a device on your LAN.
Your WAN (Internet facing) IP is not static, it's dynamic and likely to change whenever the router (hub) is restarted or the connection refreshed.
Unless things have changed recently I don't think BT have active IPv6 specific DNS servers running yet. Alternate DNS servers to try if you like, you just set them on the individual PC/laptop etc.
I've merged the content of the new thread you created with your original thread. It's easier for the community when the conversation is all in the one place.
I'd appreciate it if you would refrain from creating duplicate threads about the same issue. Please keep the discussion on the one thread. You can find all your recent posts by clicking on your username whenever you're on the home page.