As I mentioned in my previous post I was on .613 before I performed a manual reset. Immediately after the reset, and 24.5 hours later, the version of the firmware is now 9.1.1802.610. So it has not pulled down the update, much like @appleaday
Looking back at the problem of the hub 3 resetting itself at some point over the weekend, it is feasible that it reset itself when on version .610 and then, before I had looked into it, installed the .613 update. In other words, I cannot be certain whether it was running .610 or .613 at the point that it reset itself. Now, however, I am definitely running .610 and so presumably still susceptible to an unsolicited reset.
Is there any way to force the .613 update either from my end or from the Virgin Media side?
So, I've had a response from the deployment team to confirm the situation.
I've been advised that a further change is required to update boot files, which is scheduled for next week. It does appear that, at present, a reboot or pinhole reset can revert your back to .610 rather than forcing the update.
Our specialists are looking to manually update both of your hubs to the newest firmware, so you should find that this is completed for you soon. Following the additional change next week, there should be no further issues with resetting and all customers should have the latest firmware.
Thanks Rachael. I'll hang tight for the update and come back to the thread if I still have reset issues after the update. As it has previously happened pretty infrequently it'll be hard to give a definitive "all clear", but after a decent interval of time without no resets I'll come back and report success.