analysis of costs per project

I use Instant Accounts for my marketing and PR agency.  I want to be able to attribute purchase invoices to particular projects and then run a monthly report showing the costs incurred per project.  I thought I might be able to use the Ex. Ref field in the batch invoice processing, but there is no equivalent in bank payments, and in any case I cannot find a way to filter a report using the Ex. Ref field.  can anyone suggest a way that sage can produce the report that I need?