Thanks to all that have responded to my query on how to check TARGETS values.
In the end I followed Sebastian's advice and wrote a Xquery to check them,
though Schematron is probably the way to go in the future (I just don't have the time this weekend to get my head around it).
Thanks, Syd for raising questions about the markup.
MB> <link targets="#person #p1"/>
MB> <link targets="#place #Z10001980"/>
MB> <link targets="#time #d383873384226"/>
this awkward thing is a leftover from an early design stage, when we used records in an <interpGrp> to align
*certain* occurrences of an ID with their category, but this has become redundant now.
<link type="person-place-time" targets="#p1 #Z10001980 #d383873384226"/>
we didn't want to use this solution because we have to allow for more than one person
and would have to use programming to distinguish if the value is a person a place or a time
<ptr type="person" target="#p1"/>
<ptr type="place" target="#Z10001980"/>
<ptr type="time" target="#d383873384226"/>