Well, after having no end of problems with IPv6 connectivity to external sites, I've had to completely disable all IPv6 access on my network. Anyone else having problems?
I can't get to the Azure portal for example, nor does the Microsoft store work.
As soon as I remove IPv6, everything works with no problems.
Same issue in the UK, looks like an MTU issue somewhere on HE's side. I've enabled TCP-MSS clamping down to 1370 and that is working around it for the time being, can't seem to go much higher than that.