Ok - that's a very good point about using the speedtest site as a diagnostic for this problem due to the upload component! I'll try something else like a steam download, which will easily saturate the downstream but not touch the upstream. I'm not expecting perfection, but avoiding the 1000ms+ lag spikes is what I'm after.
I'll have a play with the socket tonight. I'll just plug directly into the test socket and see if it helps much. I'll leave it like that for a few days but I will eventually need to tidy things up. If it's a success, I'll modify the connectors as suggested.
