In document setup you should have facing pages checked, and the page dimension should be 210 mm wide and 257 mm high. In the bleed setup fields enter 3mm for all (but see InDesignSecrets » Blog Archive » Breaking Pages Apart to Bleed Off a Spine) if you have any artwork that dies into the spine (crossovers will be fine as-is). The bleed at the spine on a facing pages spread (unless it is split apart) is the edge of the opposite page.
This should give you a red line around the outside tha is the bleed boundary, a black line that is the page boundary, and a magenta (or pinkish) line inside the page that is the margins you've set, which you didn't specify so I can't tell you what to enter.
Hi. Think I must have done the setup wrong - how do I go back to
change setup part way through my project to correct this please. (You are
talking to an aged person:)
As definitely my black lines have the finished page boundary 210x257mm at
You should be able to just go back to File> Document Setup... and make the corrections. Expect things to move on your pages, though, and the text to re-flow.