I enclose some snapshots, to try to illustrate what is happening. They are quite small but should be readable.
Snapshot1 Shows a first pass through an Add New record screen, with the period, room and location combos with a selection showing. The others haven't been selected as n/a, so they show they show the empty entry I get when showing the combo boxes the first time. The next snapshot shows a displayed record, where most of the combo boxes are showing a selection, as this is a record where most every field is set.
Snapshot 2 shows the next record, which only has Period, location and room set the other fields are not set, so I gtk_combo_box_set_active (to -1) to try to get back to no selection. If I set it to 0, I get the 0th entry added to the combo.
The last two (I haven't sent) just show all combo boxes as a result of a Clear_Entries() setting the active to -1 has no effect, and setting it to 0 for every box, resets every combo to the first entry.
What I hoped to achieve, was all combos could be reset to an (empty) entry on being 're-set', i.e. as they appear when first shown. But I cannot seem to get back to that condition, once a particular combo box has had a selection made.