Hi @seadowg
I found an alternative app called Bluetooth GNSS from ClearEvo.com (on the Play Store). It can handle NTRIP / RTK corrections (irrelevant for the old iBT-GPS), works as a mock-location provider and is open-source (no ads!). Works well with ODK Collect.
I noticed that using the placement-map option, the mapbox mapping is able to detect the accuracy of the external gps by putting the blue circle around the point (when I have 1m+ accuracy). So just need to get the accuracy data recorded in Collect... But being mindful of people having to cope with naughty enumerators making up their locations!
Thanks for your help.