I have resolved it - thanks for you help.

I gradually copied and pasted sections of the plan into a new plan until the issue re-appeared, and then back tracked until it didn't.

It turned out to be an item (a microwave actually) on the plan, but it showed up separate in the aerial view, which ...

No, that doesn't help. The item doesn't seem to get picked up in the plan - only the aerial view.

I can send a screen dump if that helps? Not sure how to upload them here.

My aerial view appears to contain a "rogue" item that does not appear on my plan. The item is some distance from the main plan and consequently causes the aerial view to be "zoomed out".

The item appears on the left of the aerial view, so zooming out on the plan doesn't reveal it, as this only ...