Right, so this evening, I stripped down the female connectors on both sides. The metal contacts are easy to remove by sliding out the red securing strip, hooking a small screwdriver under the retention tabs above the contacts and then pulling them out of the back of the yellow plastic housing. Then I used a pair of needle nose pliers to slightly oval the female contacts and put them back together again. The result was that the male and female connectors took quite a shove to reconnect, but were clearly making good contact. Once done, the job was jobbed.
TL;DR - loose connection - has taken many months to sort - ridiculous.