My cheap eBay USB-Midi cable arrived today, had a go at trying to update the DT25 firmware, was getting nowhere with it earlier, kept getting mid error. Tried rebooting the PC, the amp, putting the amp in safe mode, trying different USB ports, wouldn't have it at all.
After reading on the forums, I ordered a better cable off Amazon for around £23.
Just came back and gave it another go, don't know why, but I thought I'd give a try to swapping the midi cabled around, I plugged the cable marked in into the out on the amp and the out on the cable to the in on the amp.
Worked first time
Just thought I'd put it out there in case anyone else was having the same problem, from what I've read it's quite a common issue.