Sarah,

Relatively simple solution - just one line of code.

In includes/modules/payment/worldpay.php at around line 123 find this section of code:
Code:
      $process_button_string = 
      zen_draw_hidden_field('instId', MODULE_PAYMENT_WORLDPAY_ID) .
Add a new line immediately below this as follows:
Code:
zen_draw_hidden_field('accId1', 'x') .
Replace the 'x' with the appropriate account number.

That's it!

I haven't tested this so test it out first yourself before using live. Please report back here whether it works or not.

Regards,

Alan