I have found my own answer to the question I asked earlier:
\setuppapersize[A4]
\setuplabeltext [en] [chapter=~]
\define[2]\MyChapter
{\framed[frame=off,width=broad,align=middle]{#1\blank[2cm]#2}}
\setuphead [chapter] [command=\MyChapter]
\chapter{Chapter title}
It was simply a matter of working out where to put the \blank[2cm] (or
whatever vertical space was required). It replaced {#1\\#2}}
Julian
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