Oh my god, I fixed it.
but it's weird...
So I looked for sling home plugs, couldn't find them so thought I'd upgrade anyway and got some ZyXEL 600mbps though plug ones which arrived today, so plugged them in, and still no iTunes server.
Undaunted by this set back, I decided to idiot check my whole router/switch set up and test everything again.
When I did my long cat5 cable check before, I didn't actually plug into the router as the router was difficult to access, I went into the powered switch (my router only has three Ethernet points, one for my readynas- running the firefly server- one for the home plug and one for the switch). So I thought if it worked in the powered switch, it would be the same as (if not worse as it's one link down the chain) plugging into the router.
And that was my mistake.
If the home plug is plugged into the router along side the firefly server, the soundbridge can't see the iTunes server.
If the home plug is plugged into a powered switch which is plugged into a port on the router, it CAN see the iTunes server!
This seemed completely counter intuitive to me, but what the heck, I'm happy and, though I might have been able to work that out without buying new homeplugs, but what the heck they are an welcome upgrade.
Thanks or all your help and suggestions, everybody. The worst thing would've been silence (my thread on the readynas forum remains unanswered) and you've been great. I wouldn't have got there without you.