After a bit more investigation it seems that there is something in some topographical surveys, but not all, that I am supplied with that produces this issue. The problem base plans initially open OK with AutoCAD 2005, but once opened with TC and saved then have proxy objects that AC2005 complains about. This is not a problem with all topo surveys and doesn't arise at all with many drawings. There does not appear (to me at any rate) to be any immediately obvious cause for this. Various drawings to which I have added a great deal of detail, e.g surveys of over 1000 trees using many paper spaces or complicated landscape schemes do not cause it. Some relatively simple drawings do. It doesn't seem to be related to using either ordinary or multi-line text or both, and the use of bezier curves does not always cause it. It doesn't seem to relate to the use of hatches either, some drawings with a lot of hatching open fine in AC2005; others with none or very little cause the message about proxy objects.
Does anyone know if any of AutoCAD's object enablers would deal with this? AC05 reports that it needs ExtcadCustObjs, but the only single page that Google brings up when I search for this is a dxf file. Where can I get ExtcadCustObjs?