The cable tester I ordered at around 17.00 yesterday arrived in this morning’s post. And jolly good it is too for such a ridiculously cheap item. As expected, the instructions are amusing but pretty clear for all that. It is easy to use and feels fairly robust, despite the price.
Now the results. I am an idiot. On testing it turns out that there is nothing wrong with the RJ45 coupler I blamed earlier. All the pins are correctly wired straight through. What /was/ at fault was one of the two cables I had joined with that coupler. One was fine, the other was only wired at 1,2,3 and 6 (so two pairs, not four as required by 1000baseT – and as pointed out by David). When I removed the coupler I had also removed the offending cable and ended up blaming the wrong item.
In my defence I plead poor eyesight exacerbated by poor ambient lighting underneath my desk. That is why I did not spot the incorrect wiring at the time.