I'm a new user to Creator and have stated using Deluge scripts - which are proving challenging!

I'm migrating an old legacy system I created 10-15 years ago to Creator. The current system collects essential banking information account name, bank sort code, bank account number, bank name and amount to allow me to submit a flat file to the Banking Clearing System so that I can collect money from out clients bank account. I have imported all of the UK banking information into a Creator Form and List. I'd like to use this banking information to pre-fill 1 field based on the name of the bank - effectively using the bank name as a key filed to look up the correct sort code.

I'm using a 'lookup' field like this which allows the user to start typing the bank name (supplied by the user) which narrows down the 18,000 possible bank branches in the UK. This field obviously displays the bank name and sort code to allow the user to identify their bank by location then bank name. I'd like to use this field entry to complete the next 2 fields which are 'bank name' & 'bank sort code' - this acts as a data validation check thus reducing user input errors. The the user can simple enter their .bank account number' thus capturing the essential bank details. After 2 days I've been unable to achieve this. Can it be done? is there a better way of achieving this?