I can believe that humans are involved in the process, but I find the explaination she gave me hard to believe, specifically that someone manually entered the text on the return form after the cheque was rejected, therefore the wrong cheque number on the return form is not the cause of the cheque getting rejected.
I think it's far more likely that someone entered the wrong cheque number initially, and after the cheque was rejected on the UK side, the return form is printed using the wrong cheque number already in the system. After all it's a standard form, with my name, address, bank account number, bank branch address, and other info. You don't think they need manually enter the above info for every return form, do you ?