Update on 'progress' :-).
Checked the ethernet cables, and the TP-Link 5 port Gigabit desktop switch box (they left me with) by plugging work laptop into them and connecting and that worked fine. Also in the hub manager I can see the NVR is connected, has an IP address, (always use this address is set to No).
NVR box is housed in a box on the side of the shed where the cameras are. Should I get it out and bring it back to the house and connect it from within the house to see if that makes any difference?
Worth a try - bringing it close to the router and try a couple of different Ethernet cables just in case.
Does the router show what speed the box is connected at?
Did you try with the switch connected to the box with a short Ethernet cable and connected to the router with the 80+m cable at the same time? Did you do the same with the Laptop?