Customisation help - 'exclusive' check box

On customer entry form->contacts sheet there are check boxes for primary billing, shipping etc

These are not ‘real’ boolean databound fields on the contact table, but change a value in the customer table like the Customer.PrimBCon field via code

When you check the field for one contact it is unchecked for another, if already checked.
I would like to replicate this behaviour as a form customisation.

I have added a new UD field the customer table to store a contact number

I have added an unbound column to the ‘Contact’ epi dataview on the customer form and exposed it on the form bound to a check box control.

Now I have got a bit stuck - if i toggle the field it triggers the EpiViewNotification event i have created, but I can’t seem to access the value of the unbound column.

Anyone tried to replicate this type of form UI behaviour? Is there a better way to achieve it?