Hi,
There is nothing to check in the code of the plugin. FTP details won't help. The plugin IS working.
As I said, if you get that error it means that you're not entering the correct information in the merchant number and the CPI store key fields. From the documentation:
Merchant number : The merchant number that HSBC should have given you. It's a 13-character string in the form of UK12345678CUR.
CPI Store Key : Put here the Key Store you should have received from HSBC. This is a 32-character string with letters and figures.
Please contact HSBC to confirm these values. I cannot tell you myself if they are correct or not since I'm not your bank.