Sorry, I had missed this question. I'm using the Superhub in modem mode with a TP-Link Deco P9 mesh system. Unfortunately, the Deco router configuration options are very limited, and I'm not sure I can alter the IPv4 firewall settings, except to uncheck the option "Ignore ping from WLAN".
Incidentally, I upgraded to the 200Mb option in mid-September. For one day only I saw a speed of over 212mb down, 22mb up. For the past two months I've only once seen >100mb down or >20Mb up; else speeds range from 55-93 down and 5-11 up. I test on laptop and phone, near the base router, when there is minimal other use of devices (kids are in bed etc). I've used the fault checker tool on the VM website which says there's no problem.
sorry, please ignore the last email, as I eventually got through to the helpline. Helpfully, if a little frustratingly, they (Joe) readily acknowledged that there has been an issue in the area since 28 September, with an estimated fix date of 15 December at 10:15.
I have no idea why this couldn't be communicated proactively, or made known to you as a community expert, or acknowldedged when I bother to run the service status diagnostic test (https://my.virginmedia.com/faults/prepare-perform-diagnosis) which tells me that there is "no issue" generally, amd specifically "no issue in your area" (image below).
Anyway, once we're past the 15th I'll do a factory reset on the SH3, test eveything and reinstall my Mesh system. Hopefully that will mean all issues are fixed, but I guess we'll see. Thanks again for your help.