ssh into RasPi "no route to host"

Yes, the speed, and yes the lights: green for 1G and amber for 100M.

The cable was the first thing I suspected, so I did try short lengths of cat6, over the carpet of course, because there's no flat cat6, I think. Same lost pings. But the flat cable worked just fine between two units if I bypassed the router with a cable joiner (and set fixed IP addys). Lost pings also between two Raspberry Pis via the router at 100M.

1000BASE-T was originally specced to run over 100m of the original cat5 cable (not cat5e or cat6), suitable for existing installations in large businesses, but it did push cat5's safety margin somewhat and it would become unreliable as the plugs oxidised and the cables aged. So cat5e and then cat6 tightened the spec to increase the reliability.

But there should be no problem for newish cat5e over the relatively short runs of a home installation for 1G. The flat cable that I bought (forget where) is actually marked "Cat.5E Verified for gigabit Ethernet".

They get away with it because most home users will not notice it and if they do, it's unlikely that many people would go OCD on locating the problem like me. It doesn't affect Internet access, only communication between units on the Ethernet side. When I first noticed that the file-manager connected to my network share would freeze for half a minute (its own timeout before retrying) if I didn't keep accessing it, I initially assumed that it was a Samba issue. Most people would probably put up with it.

I did consider reporting it on Virginmedia's Superhub support forum, but then I saw that it was filled with people reporting more dire problems with the Superhub, and anything complex and technical from me would probably be met with incomprehension. Support techs are not going to be engineers, and I'd have to recommend that it be passed to their engineering department for investigation, and then to the manufacturer. I decided not to bother. It works for me now.

I've got setup scripts that I run on new installations of Linux, because I reinstall Linux from scratch on my computer every few years after it accumulates too many unused files after several distro upgrades. So I've just added a command to ping the router every 9s, and included it in my setup script. So I'm happy to leave it as it is for now.