Thanks John!

This works perfectly on my end but sadly I can't use VBA UDFs client-side, and I want to use it as a validation formula in Data Validation which doesn't allow UDFs, so I've had to redesign:
  • Amended the database procedure to add a comma between values in the strings in Column B
  • Added =ISNUMBER(SEARCH(SUBSTITUTE(C2,",","*"),B2))="TRUE" into Data Validation
  • Require users to put a comma in if they are typing a string, and require values to be listed in the correct order.
This shouldn't actually be an issue as 99.9% of the time they will be filling column C by either copy/pasting from column B and deleting out unwanted values, or using a form with check-boxes that generates the string for them.

Thank you for your help!