Thanks for advice Chris, however, having tested web apps with both British National Grid and WGS84 data I can confirm that the coordinate widget tool works with data in both projections and even in apps where data has erroneously been added in both projections simultaneously.
The issue we are seeing is user-specific, in that we have produced an app for all members of staff that they are unable to edit. Some of them see the coordinate widget while others do not and we are still unable to determine why this is.