OK, progress has been made! I disconnected the modem and connected a switch to that end. Still 100. However I then went to the router end instead and connected that end of the cable to the switch (so: modem - 50m cable - switch) and it came straight on at 1000 according to the lights. With a short cable between the switch and the Vigor, the connection is now at 1000 all the way.

So from what I can conclude, the router does not like something about that cable, but the modem and switch are OK with it. In time I'll probably replace the cable with a Cat6A, but for now I'll just leave the switch there to get round it.

Macavity wrote: I've come across it with ISP cable modems from time to time.

It's a Hitron CGNV4 provided by VM Business that I have, so I'm probably in the same boat.

OK so I'm back at square one now, as I noticed this morning that the connection between the switch and the modem has decided to drop down from 1000 to 100, for whatever reason.

Rearranged both ends from T-568B to T-568A and all is well. Always 1000M even without the switch in place. Can't explain it but it's all working