I got this to work perfectly now I've watched the video properly.

BUT

if I get to the checkout having correctly evaluated the shipping and use the browsers back button the shipping value is re-calculated with an additional item added.

So with a cart containing 4 items it now thinks there are 5 and there seems to be no way to reset it unless I go right back to the checkout and click 'update cart' which seem to wipe the cached value.

Any one else seen this?

Any one fixed this?