When I have a really large object, and a really small one in the same scene, the small object disappears in the viewport - or is only visible at far distances. I delete the large object and then the small one re-appears??

Any ideas?

Sounds like a clipping plane issue....have you tried adjusting the viewport clipping planes. You do this by right clicking on the viewport name and selecting viewport clipping. Then use the 2 triangles on the verticle line that appears on the right of your viewport to adjust the near and far clipping plane.
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Dave