I thought the name servers are valid. My dns shows a record for ns1.badbison.co.uk pointing to my public ip address which the router points all ports to the same ports on the lan ip of the server.
As the second name server I've used ns10.schlundtech.de as that is used in the howto. To get the ip address of this server I used the dig command which returned in the answer section 18.104.22.168. I have put this address in the ip address to export zone records section of my dns records.
That sort of refers back to my earlier question of how does ns10.schlundtech.de know it has to act as my secondary name server and go get the records.
I can't display the contents of resolv.conf as I'm on a mobile device right now and the result get lost out of the screen, although you're welcome to ssh it yourself, though I appreciate you can't help that personally to everyone.
The fault does seem intermittent. On my local network I can view web pages held fine. It also works from my phone fine using the telephone connection. It worked fine from work yesterday, but today it does not. The template sites I've put up are badbison.com and prizepalace.co.uk if you'd like to view.
There is something playing on my mind though, I have a faint memory of Reading somewhere not to use the same domain name as your LAN is. My router does have the same LAN domain as my name servers domain - badbison.co.uk. Have I imagined this or should I change the routers domain setting?