Richie,
By complete coincidence, John and I both got 4mm rollers from GW Models at the same time.
Simple, elegant, solid mechanical engineering at its best. c.Stg £75 inclusive of post, and worth every penny.
GW also does a rivetting machine (different sizes and a platform to do it in a straight line quite rapidly), and I fear the Gentleman's account may be hit for this as well in the near future...