OK, I still don't understand the issue with my cat5 cable, maybe I just got a duff one.
Anyway, once I reflashed to OpenPeak, then Telnetted to install PnPIII things were looking up, it worked just like my other Joggler, but without the wired connection and with poor signal in the bedroom I thought that was it.
However, I retried the powerline network extender and this time it just worked!
With Joggler number three arriving soon I'm very glad to have got this one sorted, knowing that I am able to set them up as I wish.
I guess KISS applies, I'd got into all sorts of blind alleys trying to use SSH to get PnPIII on the joggler after using Birdslikewires to flash to SQPOS.
At least I've learnt a few more things about using the Joggler!