Further thanks to all.
Since writing I have double checked firmware and drivers – Cougar’s link was where I think I did this when I first set up the dock, and both said ‘you are already up to date’
Power – good point DaveP. 90W supply (came with dock), vs 40W on the laptop – swapped and got an error message about low power. USB and network all behaving as expected, just the display ports (two of them: neither works).
I did have a DP-HDMI, and swapped if for DP-DVI (thinking it was the cable). Same symptoms (display not detected). The dock has only DP outs. Monitor works with HDMI-HDMI. I don’t have another DP source to test with unfortunately.
It’s not one of those directional cable things is it? I thought that was just for HiFi buffs?
Lenovo forum suggests drivers, but I think I’ve addressed that. Have logged a ticket.