Hi Nick,
To my mind it's better to do this way:
It's a traditional asymmetric report, lets have column axis static. You can put somewhere on the sheet cells with current period and 2 categories (ACTUAL an BUDGET). In the cells you can use formulas like EPMSelectMember.
Then enable member recognition and in colomn axis cells insert links to the mentioned cells (for prev year use =left(B1,4)-1&right(B1,3) to generate prev year period). The links will be recognized as column axis members and the link will be inside member formula. You will be able to change members in the original cells and report will change to another period or budget version.
B.R. Vadim