Having monitored the SH-2 log for a few weeks would appear that the PPP session drop out/reconnection is every 12 to 14 days, now inclined to think that it's not a Hub re-boot as originally reported but just a loss/renewal of PPP session.
Rather than over complicate logging, have just periodically monitored/recorded the SH-2 "DSL Uptime" from it's GUI, it's then quite easy to scroll back the "event log" filtered by PPP to establish exactly when the session drop out/reconnection occured.
Have only caught the Hub "in the act" of reconnecting once, the time of day appears to be quite random.
Obviously with FTTP there is no real impact to the system, reconnection is virtually instantenous, can't get overly excited, just curious!
Never been in a position to monitor the ONT LED status indications during a drop out/reconnection.