The MWE was really just the only line affecting headers, which you 
suggested should be written as it is now written below, rather than the 
way I had it earlier (a leftover from a lack of understanding about font 
switches which Otared kindly explained to me).

\setupheader [style={\switchtobodyfont[10pt,ss]}]

This proper way of setting out the \setupheader has in fact resolved the 
situation and it now applies to both recto and verso headers. Thank you
Julian

On 4/9/20 7:16 am, Aditya Mahajan wrote:
> On Fri, 4 Sep 2020, jbf wrote:
>
>> Thanks Otared. That clears up that little item :-)
>>
>> BTW I know how to reduce font size in the headers (chapter titles) that
>> appear on the recto pages, which I can do with something like
>> \setupheader [style={\ss\switchtobodyfont[10pt]}], but it doesn't seem
> Better to use style={\switchtobodyfont[10pt,ss]} or \switchtobodyfont[10pt]\ss...
>
>> to affect the header (title of the book) on the verso pages, which is
>> drawn from the simple statement:
>>
>> \startdocument
>>     [metadata:title={Book title}]
>>
>> Shouldn't  \setupheader affect both headers? Or is it possible I have
>> left something out of my \setupheader declaration?
> A minimal example will make the debugging process much easier.
>
> Aditya
>
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