[...]

Since I posted this a short while back, I feel it is only fair to post an update since the response from Bitmain has been reasonably impressive.

After a few days of initial troubleshooting (back and forth communication with Bitmain where they asked me to try various things to fix the problem), they asked me to send the units to their repair centre in Europe. I know from following the tracking number that the faulty units arrived at the repair centre on Wednesday 3rd June. On Thursday 4th June I received a notification from Fedex of 2 brand new replacement units being shipped from China, which duly arrived here on Tuesday 9th June. They would have been here Monday 8th if it wasn't for a delay in customs.

The replacement units are working perfectly. Whilst it's not good that I received two faulty units initially, in their credit they have replaced them very quickly and with no complications.