Thanks for your reply, but I think you misunderstood me. I'm fine with the selections, but need to alter the expressions, depending on what those selections are. As a simple example:
If the user selects June 2012, the expression for a particular column might be CYM1+CYM2+CYM3 (returning the last three months). But if they choose April 2012, the expression for that same column woiuld need to be CYM3+CYM4+CYM5 (the three months up to April). CYM1, CYM2 etc are columns in my fact table.
I'm trying to avoid long, messy IF statements.