Yes it was directly in front of the router - it had to be because I had to run a short cable from the router to connect by cable temporarily. Firewalls hadn't changed, but tried it without - no difference :-(. I experimented with DHCP & assigning addresses. No joy.
So....
I had a Pi 3b+ doing another job, and I swapped the brand new LE card into it - connected to the network 1st time. I tried the original LE card in it. Connected to the network 1st time.
I thought then it must be a the original pi 3b must have got a fault. I swapped the machine that was doing Libreelec into the job the other pi was doing, and it worked 1st time with no changes.
Grasping at straws, I wonder if the "bios" of the pi 3b that I was having problems with LE with, was not as up to date and couldn't cope with something the router was doing.
Whatever - it now works.
Case closed.