GS724T affecting internet upload speed??

Can anyone help with this please.

I have BT Infinity 2 with a Home Hub 5 router/modem.

I am getting excellent download speeds (70+ Mbps) but dreadful upload speeds (2 Mbps) when my PC is connected to my home network ie through my GS724T switch.

However if I connect my PC directly to the router all is well (speeds of 15 Mbps). And I get the same speeds if I connect wirelessly.

So BT (probably quite rightly) say its an issue with the switch or the cables.

I cannot see how its the cables as surely if they were faulty it would affect download speeds as well.

If so I am assuming that its a configuration on the switch I have not set correctly.

Can anyone suggest what I need to do? And I am not hugely technical so some hand holding would be appreciated.

Many thanks