This is starting to make sense! Synetrix put a change on their firewall a few weeks ago to allow '5 on demand' traffic. We quickly found that the change broke parent pay and our cashless catering boxes. Synetrix put a permanent fix in this morning to address that which seems to fixed this issue as well (something to do with x-content headers - whatever they are!). We've running through our 16 calls to check all is well, 5 and counting with no issues!
Unfortunately Phil, Capita now owns Synetrix so I can say with absolute certainty that it was Capita's fault all along!!