OK, hacking attempt problem now sorted (apparently some Linux kernel modules weren't installed on the new server), but now whenever it tries to connect to the CPI, it comes back to hsbc_return.php with an "invalid input data" message-after reading back through this thread it seems the problem was often related to HSBC knowing that the site is running in production mode, but it has been for over 12 months now so that shouldn't be a problem. Any ideas/thoughts would be much appreciated.
Cheers, Jon