Pricing Search - Filter by Customer

I am trying to set up a Pricing Search so that I can produce a Price List for Customers in a suitable corporate format. (The default functionality for creating a customer price list has very few options for customising) .

Using the Support Help topics, I can create a search that is filtered in the Criteria as 'Customer is' and select a customer and this works fine.

But if I then put the Customer Field in as a Filter on that search, it cannot be changed - in the drop down I only get options for All, Unassigned, Mine, My Team, Mine and Unassigned, My Team and Unassigned, and then the customer I defined in the Criteria is shown at the bottom of the drop down. The field will not allow manual typing, so I cannot select another customer.