I'm reasonably sure that the issue that I have is a software bug. It didn't used to happen, the line stats have remained the same, I can only think that a problem go introduced on a code update to the HUB 5 or perhaps an update to the DSLAM, that conflicts with the hub under certain circumstances.
It is important to remember that the PPPoE disconnection is likely not the root issue. More likely, it is a symptom of a deeper problem.
It is normal for the end initiating a PPP connection (in your case, the HH5) to send periodic LCP echo requests - like a ping, but at a lower level of the protocol stack. If several of these LCP echo requests fail to produce a reply, the PPP connection is torn down (a disconnection) and re-established.
It's also possible for the end terminating the PPP connection to tear down the connection. This can happen if the termination point notices the connection is dead because traffic isn't being responded to, or for management reasons such as the ISP rebalancing load between its gateways.
The PPP disconnection might be because there is a temporary drop out of the DSL - maybe not enough to knock it out of sync, but enough to cause a break in data transmission. If the router shows statistics for errored seconds and severely errored seconds, that might help in understanding what is wrong.
Replacing your router with something different is a reasonable way ahead, as it eliminates possible causes at your end. However, if replacing the router cures the problem, that does not necessary mean there was a software bug - it might be that there was an incipient hardware failure.
Paul - it's usual to use PPPoA over ADSL / ADSL2 / ADSL2+ and PPPoE over VDSL2. The operation of the PPP layer is the same, even though the layers underneath work differently.