The problem turned out to be corruption (or unexplained changes) to the tab leader in several style definitions.

Basically the tab leader was changed from non to dots for all tabs except the first.

Given the number of clicks you have to go through to change the tab leader I would have remembered if I'd done this manually.

There is a slight chance there is a macro error somewhere in the routines that retrieve style data from a spreadsheet.

BUT this is the same document that suddenly started showing sequential numbers between the heading number and the tab preceding the heading text (of the form XXXB)

i.e

3.2.P.3.3<tab>Heading text

became

3.2.P.3.3<tab>XXXBHeading text.

The additional numbers were initially visible only in the navigation pane.

They weren't visible in the header itself until I reapplied the same header style. If I changed the header style to another these numbered remained hidden (but visible in the navigation pane), but if I reapplied the new style then they did appear.

The numbers also appeared in the heading if I put the cursor at the start of the heading text and backspaced (which showed there was also an indent or tab associated with mysterious numbering).

I'm now curious as to what other anomalies I might find further down the line.