Just a follow up, as it may be useful to somebody. I continued to fiddle with my hub this evening, and found it got into a state as I went through the config pages where it seemed to get a little confused about whether I'd changed settings or not and kept insisting I should save my changes over and over again. I went through the loop with it a few times, and then out of concern said no and asked it to reboot. It took a little while to come back, - but when it did I seemingly now have a working L2TP connection via the Hub and have been able to spend a couple of hours using it. Hooray! It's a little concerning that I still have no idea why it now is working (as I absolutely have done nothing new to warrant this), and I did have previous bursts of seeing it 'kind of' work for short periods of time previously, which it then gave up on subsequently, so who knows if it'll last. Thought I'd mention it, all the same.
The main changes I had made had been turning the firewall off, and back on, putting my machine in the DMZ zone, and then cancelling it, and looking at the port forwarding pages (although I didn't actually make any changes there). I did experiment with changing the MTU value to 1400 (following advice from another post) and then put it back to 1500.
I’m also having this issue on superhub 2. It had been working fine for a couple of years but I had to do a factory reset and am now unable to accesvia l2tp. PPTP is working fine on my windows laptop but that isn’t an option for iOS. If anyone at Virgin wants further information I am happy to provide it.