Aha that could be it. Interested to know which model of switch gomezz is using which works fine.
I bought this cheap-as-chips ES back in January - works flawlessly allowing the YV 4K Box and my Samsung 4K TV to use just the one cable to my BT HH5 Router ..
Edimax ES-3305P V3 5 Port Desktop Ethernet Switch
I agree with Keith_Beddoe that that's a possible explanation. It seems weird that Netgear make the two switches with little price differential. So it's about something other than managed/unmanaged.
The difference is that consumer grade switches are simpler, as they are intended home use, whereas the others are aimed at business customers who need the extra features.
The GS305 is only £2 more than the GS205. The managed, at £25, is more than double the cost of the GS205 so I still ask why Netgear bother with the GS205. It only leads to confusion as we see here!
In the early days of home networking, the only devices that were affordable were Ethernet hubs. These were very dumb devices that replicated all traffic on every port. There was no flow control, so quite often you got collisions where more than one device transmitted at the same time. They used to have a "COL" LED on them to indicate when this happened.
I had a number of hubs, and they were quite useful for traffic monitoring to see what was going on within the network.
They are all gone now, and been replaced with fully managed switches.
So we now have three types left.
Basic unmanaged ports which are very cheap, and suitable for general use. (GS205)
Smart Ethernet switches which look at layer 2, and do clever stuff with QOS and multicast. These have no management facilities as the features are locked in firmware. They cost a little bit more (GS305).
Fully managed switches like the GS105E do the same as a smart switch, but have full web management.
Each port can be configured in order of priority. Rate limiting can be set for each port to stop one user hogging all of the bandwidth.
Sections can be configured as part of a VLAN, so you can segregate traffic. For example you could have a number of devices connected to the switch that can all communicate with each other, but only one has Internet access.
You can also have guest connection which only has access to the Internet, but not to any other device on you network.
I only use fully managed Netgear switches here, so I have full visibility and can run network diagnostics.
Its possible that the GS305 may well suit @infaddict, but I have no experience of that device, which is why I suggested one of the ones that I use.
Some of the managed facilities of the GS105E, may come in useful if you want to give a gaming console priority over a Now TV box, or similar device, provided they are all connected to the same managed switch.
The new managed switch arrived. However, before I opened it I thought I'd try everything again from scratch using the original unmanaged switch. To cut a long story short, I got it working!
I think the issue was in the boot order of the devices connected to the switch. I don't understand how or why but it only seems to work properly if the YouView box is the first thing connected, then rebooted. After than I can connect other devices and it all works fine.
So I wanted to say thank you for all the help and advice, this is a great forum! And secondly wanted to say sorry for wasting your time, but I've learned a lot and have all 3 devices now working through my switch! I'll return the managed one back to Amazon for a refund.
Don't think you have wasted our time as most of us have learnt something from this.