Rebooted it and momentarily regained sync and it logged in, then dropped, resynced etc.
Rang Zen who told me it had been down since about midday and they were seeing sync, auth, drop every couple of minutes since the reboot.
Swapped the router for my backup Netgear DGND3300, connected to the test socket and replaced the filter for a spare ADSL Nation/Tandy one (I have two of these)
Got a stable connection for a few hours but got up today to find it synced but PPP down.
Rang Zen again who said circuit had been in sync overnight but PPP had dropped and not recovered since 11 p.m. ish.
Recommended I change the RJ-11 cable so swapped it for a longer Tandy twisted pair one I had and router has been online for nearly two hours now.
Anyone else had an issue where the cable was found to be the cause as at the moment I'm not sure whether to raise a fault with Billion as the router is 2 months old.
Router: Billion 7800N
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.