We are making a small memorial out of marble with a plaque for a friend and we need to glue it altogether.  What the best glue to use that will hold marble together and withstand the great british outdors?

I would say that you should speak to a company that makes headstones and memorials etc, to see what they use. You could also speak to a company that fits/restores marble fireplaces or kitchen worktops. I would guess that it would be an epoxy of some sort.
As CW said you should also try and reinforce any joints with rods to make extra sure things stay together.