Right, got a resolution to this one, albeit one that raises more questions than answers....
I chatted with a CS agent yesterday on the LiveChat (full marks to GE here- the chat was answered in less than 5 minutes and a resolution effected immediately). The agent (no name given) said that a setting had been applied at their end which limited the battery discharge to 55%. I queried this as I had not requested this and no setting was in my web portal. The agent said that the change had been made in GE and that he'd have to investigate it as it had no notes or information to say why it had been set.
Last night I disabled the Timed Charge to see if the battery would work as normal. The battery discharged down to 22% and then a out-of-normal-hours Intelligent Octopus period allowed me to start re-charging it.
i doubt if I will get any feedback as to why this change was made on the internal GS systems. It all sounds a bit odd, to be honest.
However, it's working as intended (and I'm glad that I didn't see any immediate discharge to zero events).

