Thank you all for putting up with my ignorance over the past while.
I originally thought that the VU+ was setting its address (and the gateway address) for other devices to connect to.. acting as a server of sorts.
I only realised yesterday that the linux PC , as you say, is acting like a router. The mistake was entering the IP and gateway addresses into the other devices rather than using dchp. This was connecting them to the PC rather than to the VU+.
So tonight I connected the Debian laptop , wired through, with the ipv4 connection set to "automatic" rather than "shared" which solved the problem. I now assume that "automatic" is equivalent to dchp.
I then reset up Kodi on the raspi , wired,with the ipv4 connection set to dhcp to make the connection, with the VU+ ip address entered into the enigma 2 PVR client. This also worked
I can now stream the same, or even different, channels on the PC and the raspi (or laptop) .
I even switched off the PC after the connections were set up , and was still able to stream. Maybe it needs to be switched on to set up the connections in the first place, I dont know. I will experiment.
Anyway people, at least I have a working network. Thanks again for all your help.
The other option I have seems to be going down the router route. The lack of cabled internet coming into the house seems it might be a problem as any I have seen so far seem to want to use an WAN input and then ethernet or wireless output. I havent seen one with wireless input yet. That problem is for another day.