Well, jimi is at his parents this week, and decided that he would try the new FireBrick PPPoE code at home. So upgrading software, remotely by many miles, reconfiguring for PPPoE on FireBrick and then on router and hoping for the best.
What can I say - it worked.
Well, technically it did not quite work, but worked well enough for us to fix a couple of slight buglets. But he is now on-line without having to drive back home.
I think he was very brave! But it does now mean we have the confidence to talk customers through this.
He also has native IPv6 "just working"® now.