Works a treat..... if you've got the email addresses.
Yeah, only if you've got the email address can you add the unique bit before.
I know it's not allowed but I've got 4 that wanted to do GCL but didn't use email.
They have me their bank details and I get the email that they would've got.
You could give them the reference you want them to use?
"Mrs Milne, this is your unique customer code, when you sign up, please enter it in the unique customer reference bit. Don't lose it. And don't forget to use it either, I wouldn't want to miss you off the 'paid' list. You only need to do it once. After that it couldn't be simpler!"
Or something like that...