Right, I've updated all my printer drivers (HP5520).

The problem persists.

If I just print page 1 of the document, it prints out as correct 12cm x 12cm squares.

If I print the whole document (two pages) single-sided, they're again correct.

But if I print double-sided (whether flipping on long OR short side), they shrink to 11cm x 11cm. - so the problem clearly lies in the 'flip'.

I think I'm just going to have to stick with the workaround: have a 12x12 template for a single-sided insert, and a 13x13 one (that actually prints as 12x12) for double-sided.