Following the steps in the post above, the formula works ... but only if a single item is selected. The lookup column in question that I have may have multiple selections made, so I need all of them to save (which currently doesn't work).
Additionally, when I go back in to a record, the results saved in the 'new' form aren't retained in the 'edit' form as so have to be selected again.
If anyone can help it will be greatly appreciated. I'm really stuck on this one.