Bypassing powerline adapters, did you test if there is any problem when two PCs or more are connected to the switch?
Homehub 5(LAN)----(LAN)Switch(LANs)----PC1 & PC2...

Another set of plugs are also TP-LINK or a different brand?
Connecting the second port, as you said, internet stops working. Can you check status of local area connection on your PC and login your router page to have a check?

Hi Vivian,
Thanks for your response.
However I think it is solved.
I have 1 pair of plugs/powerline adapters with 3 ports. If I plug 1 into my TV and one into my switch all is fine.
If I then plug in another item, say a PVR into the second port, say port 2 and then go back to the plug that is near my router and attach a further cable from port 2 to the switch the internet fails.
I believe this is causing some sort of conflict.
This happens with both makes of plug.
However what I didn't realise is that I can plug 3 items at one end. For instance TV, PVR and Slingbox, but I only need to plug one cable from my other plug into my switch.
This works a treat. I have now also plugged the other 3 port plug into a different plug point and again it works fine, as long as I only have one cable going into my switch at the other end.
Does this make sense?