Stumbled over a case with files containing data at different timepoints. Each file gives a 1 time dimension (owing to netcdf representation) : Making the time coordinates *scalar allows these all to merge correctly. However, the original data will not concatenate (which ought to be an equivalent operation).
Not yet found what may be causing this.
N.B. example data, but unfortunately large, attached to internal issue : EPM-691
TODO: chase this up + produce testcase with small data.