Actually, I used a hacked-up plastic telecoms insertion tool. It seemed to (not quite) work. What I was hoping is that, from the symptoms, someone could tell me which colour I have to repunch. I could go and pick up a cheaper knock-off Krone part, which I might have more success with -- there's a Maplin in town.

It's single-insulated solid-core cable (with an outer sheath insulating all of the pairs) -- "Acolan CTD100 4 pair 24 AWG UTP Category 5 PVC Grey, product number M0880A" -- it says on the box.

I don't quite understand the instructions for the loop-back test. Could you elaborate?