Check whether a membership number has already been assigned

[RedCrossIuK]

I have a "Members" table and there is a [Membership Number] field. The membership number is an ID and also a label. I want the user to be able to enter any five-digit member number when creating a new member record, but the entry will only be accepted if the number does not already exist. If he enters a number that has already been assigned, he should receive the message “Number not permitted - already assigned”. Can someone please help me what the Valid If code should look like for this to work? Thanks Stephan (RedCrossIuK)